Copyright: Copyright © 2001 Borland Software Corporation.                      All rights reserved.                                                 This Help file was compiled on   Friday, May 18, 2001 15:42:45.


Using Object Pascal
  Object Pascal Language Guide

Using Object Pascal

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Object Pascal is a high-level, compiled, strongly typed language that supports structured and object-oriented design. Its benefits include easy-to-read code, quick compilation, and the use of multiple unit files for modular programming.

Object Pascal has special features that support Borland’s component framework and RAD environment. For the most part, descriptions and examples in this language reference assume that you are using Object Pascal to develop applications using Borland development tools such as Delphi or Kylix.

Most developers using Borland software development tools write and compile their code in the integrated development environment (IDE). Borland development tools handle many details of setting up projects and source files, such as maintenance of dependency information among units. The product also places constraints on program organization that are not, strictly speaking, part of the Object Pascal language specification. For example, Borland development tools enforce certain file- and program-naming conventions that you can avoid if you write your programs outside of the IDE and compile them from the command prompt.

These Help topics generally assume that you are working in the IDE and that you are building applications that use the Visual Component Library (VCL) and/or Borland Component Library for Cross Platform (CLX). Occasionally, however, Borland-specific rules are distinguished from rules that apply to all Object Pascal programming.


Program organization: Overview
  Object Pascal Language Guide

Program organization

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Programs are usually divided into source-code modules called units. Each program begins with a heading, which specifies a name for the program. The heading is followed by an optional uses clause, then a block of declarations and statements. The uses clause lists units that are linked into the program; these units, which can be shared by different programs, often have uses clauses of their own.

The uses clause provides the compiler with information about dependencies among modules. Because this information is stored in the modules themselves, Object Pascal programs do not require makefiles, header files, or preprocessor “include” directives. (The Project Manager generates a makefile each time a project is loaded in the IDE, but saves these files only for project groups that include more than one project.)

For further discussion of program structure and dependencies, see Programs and units.


Pascal source files
  Object Pascal Language Guide

Pascal source files

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The compiler expects to find Pascal source code in files of three kinds:

unit source files (which end with the .pas extension)

project files (which end with the .dpr extension)

package source files (which end with the .dpk extension)

Unit source files contain most of the code in an application. Each application has a single project file and several unit files; the project file—which corresponds to the “main” program file in traditional Pascal—organizes the unit files into an application. Borland development tools automatically maintain a project file for each application.

If you are compiling a program from the command line, you can put all your source code into unit (.pas) files. But if you use the IDE to build your application, you must have a project (.dpr) file.

Package source files are similar to project files, but they are used to construct special dynamically linkable libraries called packages. For more information about packages, see Dynamic-link libraries and packages.


Other files used to build applications
  Object Pascal Language Guide

Other files used to build applications

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In addition to source-code modules, Borland products use several non-Pascal files to build applications. These files are maintained automatically and include

form files, which end with the .dfm (Delphi) or .xfm (Kylix) extension,

resource files, which end with the .res extension, and

project options files, which end with the .dof (Delphi) or .kof (Kylix) extension.

A form file is either a text file or a compiled resource file that can contain bitmaps, strings, and so forth. Each form file represents a single form, which usually corresponds to a window or dialog box in an application. The IDE allows you to view and edit form files as text, and to save form files as either text or binary. Although the default behavior is to save form files as text, they are usually not edited manually; it is more common to use Borland’s visual design tools for this purpose. Each project has at least one form, and each form has an associated unit (.pas) file that, by default, has the same name as the form file.

In addition to form files, each project uses a resource (.res) file to hold the bitmap for the application’s icon. By default, this file has the same name as the project (.dpr) file. To change an application’s icon, use the Project Options dialog.

A project options (.dof or .kof) file contains compiler and linker settings, search directories, version information, and so forth. Each project has an associated project options file with the same name as the project (.dpr) file. Usually, the options in this file are set from Project Options dialog.

Various tools in the IDE store data in files of other types. Desktop settings (.dsk or .desk) files contain information about the arrangement of windows and other configuration options; desktop settings can be project-specific or environment-wide. These files have no direct effect on compilation.


Compiler-generated files
  Object Pascal Language Guide

Compiler-generated files

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The first time you build an application or a standard dynamic-link library, the compiler produces a compiled unit .dcu (Windows) .dcu/.dpu (Linux) file for each new unit used in your project; all the .dcu (Windows) .dcu/.dpu (Linux) files in your project are then linked to create a single executable or shared library file. The first time you build a package, the compiler produces a .dcu (Windows) .dpu (Linux) file for each new unit contained in the package, and then creates both a .dcp and a package file. (For more information about libraries and packages, see Libraries and packages.) If you use the –GD switch, the linker generates a map file and a .drc file; the .drc file, which contains string resources, can be compiled into a resource file.

When you rebuild a project, individual units are not recompiled unless their source (.pas) files have changed since the last compilation, or their .dcu (Windows) .dcu/.dpu (Linux) files cannot be found, or you explicitly tell the compiler to reprocess them. In fact, it is not necessary for a unit’s source file to be present at all, as long as the compiler can find the compiled unit file.


About example programs
  Object Pascal Language Guide

Example Programs

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The examples that follow illustrate basic features of Object Pascal programming. The examples show simple Object Pascal applications that cannot be compiled from the IDE; but you can compile them from the command line.

A simple console application

A more complicated example

A Windows application


A simple console application
  Object Pascal Language Guide

A simple console application

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The program below is a simple console application that you can compile and run from the command prompt.

program Greeting;

{$APPTYPE CONSOLE}

var MyMessage: string;

begin

  MyMessage := 'Hello world!';

  Writeln(MyMessage);

end.

The first line declares a program called Greeting. The {$APPTYPE CONSOLE} directive tells the compiler that this is a console application, to be run from the command line. The next line declares a variable called MyMessage, which holds a string. (Object Pascal has genuine string data types.) The program then assigns the string “Hello world!” to the variable MyMessage, and sends the contents of MyMessage to the standard output using the Writeln procedure. (Writeln is defined implicitly in the System unit, which the compiler automatically includes in every application.)

You can type this program into a file called Greeting.pas or Greeting.dpr and compile it by entering

On Delphi: DCC32 Greeting

On Kylix: dcc Greeting

on the command line. The resulting executable prints the message “Hello world!”

Aside from its simplicity, this example differs in several important ways from programs that you are likely to write with Borland development tools. First, it is a console application. Borland development tools are typically used to write applications with graphical interfaces; hence, you would not ordinarily call Writeln. Moreover, the entire example program (save for Writeln) is in a single file. In a typical application, the program heading—the first line of the example—would be placed in a separate project file that would not contain any of the actual application logic, other than a few calls to methods defined in unit files.


A more complicated example
  Object Pascal Language Guide

A more complicated example

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The next example shows a program that is divided into two files: a project file and a unit file. The project file, which you can save as Greeting.dpr, looks like this:

program Greeting;

{$APPTYPE CONSOLE}

uses Unit1;

begin

  PrintMessage('Hello World!');

end.

The first line declares a program called Greeting, which, once again, is a console application. The uses Unit1; clause tells the compiler that Greeting includes a unit called Unit1. Finally, the program calls the PrintMessage procedure, passing to it the string “Hello World!” Where does the PrintMessage procedure come from? It’s defined in Unit1. Here’s the source code for Unit1, which you can save in a file called Unit1.pas:

unit Unit1;

interface

procedure PrintMessage(msg: string);

implementation

procedure PrintMessage(msg: string);

begin

  Writeln(msg);

end;

end.

Unit1 defines a procedure called PrintMessage that takes a single string as an argument and sends the string to the standard output. (In Pascal, routines that do not return a value are called procedures. Routines that return a value are called functions.) Notice that PrintMessage is declared twice in Unit1. The first declaration, under the reserved word interface, makes PrintMessage available to other modules (such as Greeting) that use Unit1. The second declaration, under the reserved word implementation, actually defines PrintMessage.

You can now compile Greeting from the command line by entering

On Delphi: DCC32 Greeting

On Kylix: dcc Greeting

There’s no need to include Unit1 as a command-line argument. When the compiler processes Greeting.dpr, it automatically looks for unit files that the Greeting program depends on. The resulting executable does the same thing as our first example: it prints the message “Hello world!”


A native application
  Object Pascal Language Guide

A native application

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Our next example is an application built using VCL or CLX components in the IDE. This program uses automatically generated form and resource files, so you won’t be able to compile it from the source code alone. But it illustrates important features of Object Pascal. In addition to multiple units, the program uses classes and objects

The program includes a project file and two new unit files. First, the project file:

program Greeting;  { comments are enclosed in braces }

uses

  Forms, {change the unit name to QForms on Linux}

  Unit1 in 'Unit1.pas' { the unit for Form1 },

  Unit2 in 'Unit2.pas' { the unit for Form2 };

{$R *.res}  { this directive links the project's resource file }

begin

  { calls to Application }

  Application.Initialize;

  Application.CreateForm(TForm1, Form1);

  Application.CreateForm(TForm2, Form2);

  Application.Run;

end.

Once again, our program is called Greeting. It uses three units: Forms, which is part of VCL and CLX; Unit1, which is associated with the application’s main form (Form1); and Unit2, which is associated with another form (Form2).

The program makes a series of calls to an object named Application, which is an instance of the TApplication class defined in the Forms unit. (Every project has an automatically generated Application object.) Two of these calls invoke a TApplication method named CreateForm. The first call to CreateForm creates Form1, an instance of the TForm1 class defined in Unit1. The second call to CreateForm creates Form2, an instance of the TForm2 class defined in Unit2.

Unit1 looks like this:

unit Unit1;

interface

uses  { these units are part of the Visual Component Library (VCL) }

  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;

{

On Linux, the uses clause looks like this:

uses  { these units are part of CLX }

  SysUtils, Types, Classes, QGraphics, QControls, QForms, QDialogs;

}

type

  TForm1 = class(TForm)

    Button1: TButton;

    procedure Button1Click(Sender: TObject);

  end;

var

  Form1: TForm1;

implementation

uses Unit2;  { this is where Form2 is defined }

{$R *.dfm}  { this directive links Unit1's form file }

procedure TForm1.Button1Click(Sender: TObject);

begin

  Form2.ShowModal;

end;

end.

Unit1 creates a class named TForm1 (derived from TForm) and an instance of this class, Form1. TForm1 includes a button—Button1, an instance of TButton—and a procedure named TForm1.Button1Click that is called at runtime whenever the user presses Button1. TForm1.Button1Click hides Form1 and it displays Form2 (the call to Form2.ShowModal). Form2 is defined in Unit2:

unit Unit2;

interface

uses

  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

  StdCtrls;

{

On Linux, the uses clause looks like this:

uses  { these units are part of CLX }

  SysUtils, Types, Classes, QGraphics, QControls, QForms, QDialogs;

}

type

  TForm2 = class(TForm)

    Label1: TLabel;

    CancelButton: TButton;

    procedure CancelButtonClick(Sender: TObject);

    procedure FormClose(Sender: TObject; var Action: TCloseAction);

  end;

var

  Form2: TForm2;

implementation

uses Unit1;

{$R *.dfm}

procedure TForm2.CancelButtonClick(Sender: TObject);

begin

  Form2.Close;

end;

end.

Unit2 creates a class named TForm2 and an instance of this class, Form2. TForm2 includes a button (CancelButton, an instance of TButton) and a label (Label1, an instance of TLabel). You can’t see this from the source code, but Label1 displays a caption that reads “Hello world!” The caption is defined in Form2’s form file, Unit2.dfm.

Unit2 defines one procedure. TForm2.CancelButtonClick is called at runtime whenever the user presses CancelButton; it closes Form2. This procedure (along with Unit1’s TForm1.Button1Click) is known as an event handler because it responds to events that occur while the program is running. Event handlers are assigned to specific events by the form (.dfm on Windows .xfm on Linux) files for Form1 and Form2.

When the Greeting program starts, Form1 is displayed and Form2 is invisible. (By default, only the first form created in the project file is visible at runtime. This is called the project’s main form.) When the user presses the button on Form1, Form1 disappears and is replaced by Form2, which displays the “Hello world!” greeting. When the user closes Form2 (by pressing CancelButton or the Close button on the title bar), Form1 reappears.


Programs and units: Overview
  Object Pascal Language Guide

Programs and units

Topic groups	See also

A program is constructed from source-code modules called units. Each unit is stored in its own file and compiled separately; compiled units are linked to create an application. Units allow you to

divide large programs into modules that can be edited separately.

create libraries that you can share among programs.

distribute libraries to other developers without making the source code available.

In traditional Pascal programming, all source code, including the main program, is stored in .pas files. Borland tools use a project (.dpr) file to store the “main” program, while most other source code resides in unit (.pas) files. Each application—or project—consists of a single project file and one or more unit files. (Strictly speaking, you needn’t explicitly use any units in a project, but all programs automatically use the System unit.) To build a project, the compiler needs either a source file or a compiled unit file for each unit.


Program structure and syntax: Overview
  Object Pascal Language Guide

Program structure and syntax

Topic groups	See also

A program contains

a program heading,

a uses clause (optional), and

a block of declarations and statements.

The program heading specifies a name for the program. The uses clause lists units used by the program. The block contains declarations and statements that are executed when the program runs. The IDE expects to find these three elements in a single project (.dpr) file.

The example below shows the project file for a program called Editor.

 1     program Editor;

 2

 3     uses

 4       Forms, {change to QForms in Linux}

 5       REAbout in 'REAbout.pas' {AboutBox},

 6       REMain in 'REMain.pas' {MainForm};

 7

 8     {$R *.res}

 9

10     begin

11       Application.Title := 'Text Editor';

12       Application.CreateForm(TMainForm, MainForm);

13       Application.Run;

14     end.

Line 1 contains the program heading. The uses clause is on lines 3 through 6. Line 8 is a compiler directive that links the project’s resource file into the program. Lines 10 through 14 contain the block of statements that are executed when the program runs. Finally, the project file, like all source files, ends with a period.

This is, in fact, a fairly typical project file. Project files are usually short, since most of a program’s logic resides in its unit files. Project files are generated and maintained automatically, and it is seldom necessary to edit them manually.


The program heading
  Object Pascal Language Guide

The program heading

Topic groups	See also

The program heading specifies the program’s name. It consists of the reserved word program, followed by a valid identifier, followed by a semicolon. The identifier must match the project file name. In the previous example, since the program is called Editor, the project file should be called EDITOR.dpr.

In standard Pascal, a program heading can include parameters after the program name:

program Calc(input, output);

Borland’s Object Pascal compiler ignores these parameters.


The program uses clause
  Object Pascal Language Guide

The program uses clause

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The uses clause lists units that are incorporated into the program. These units may in turn have uses clauses of their own. For more information about the uses clause, see Unit references and the uses clause.


The block
  Object Pascal Language Guide

The block

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The block contains a simple or structured statement that is executed when the program runs. In most programs, the block consists of a compound statement—bracketed between the reserved words begin and end—whose component statements are simply method calls to the project’s Application object. (Every project has an Application variable that holds an instance of TApplication, TWebApplication, or TServiceApplication.) The block can also contain declarations of constants, types, variables, procedures, and functions; these declarations must precede the statement part of the block.


Unit structure and syntax: Overview
  Object Pascal Language Guide

Unit structure and syntax

Topic groups	See also

A unit consists of types (including classes), constants, variables, and routines (functions and procedures). Each unit is defined in its own unit (.pas) file.

A unit file begins with a unit heading, which is followed by the interface, implementation, initialization, and finalization sections. The initialization and finalization sections are optional. A skeleton unit file looks like this:

unit Unit1;

interface

uses  { List of units goes here }

  { Interface section goes here }

implementation

uses  { List of units goes here }

  { Implementation section goes here }

initialization

  { Initialization section goes here }

finalization

  { Finalization section goes here }

end.

The unit must conclude with the word end followed by a period.


The unit heading
  Object Pascal Language Guide

The unit heading

Topic groups	See also

The unit heading specifies the unit’s name. It consists of the reserved word unit, followed by a valid identifier, followed by a semicolon. For applications developed using Borland tools, the identifier must match the unit file name. Thus, the unit heading

unit MainForm;

would occur in a source file called MAINFORM.pas, and the file containing the compiled unit would be MAINFORM.dcu.

Unit names must be unique within a project. Even if their unit files are in different directories, two units with the same name cannot be used in a single program.


The interface section
  Object Pascal Language Guide

The interface section

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The interface section of a unit begins with the reserved word interface and continues until the beginning of the implementation section. The interface section declares constants, types, variables, procedures, and functions that are available to clients—that is, to other units or programs that use the unit where they are declared. These entities are called public because a client can access them as if they were declared in the client itself.

The interface declaration of a procedure or function includes only the routine’s heading. The block of the procedure or function follows in the implementation section. Thus procedure and function declarations in the interface section work like forward declarations, although the forward directive isn’t used.

The interface declaration for a class must include declarations for all class members.

The interface section can include its own uses clause, which must appear immediately after the word interface. For information about the uses clause, see Unit references and the uses clause.


The implementation section
  Object Pascal Language Guide

The implementation section

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The implementation section of a unit begins with the reserved word implementation and continues until the beginning of the initialization section or, if there is no initialization section, until the end of the unit. The implementation section defines procedures and functions that are declared in the interface section. Within the implementation section, these procedures and functions may be defined and called in any order. You can omit parameter lists from public procedure and function headings when you define them in the implementation section; but if you include a parameter list, it must match the declaration in the interface section exactly.

In addition to definitions of public procedures and functions, the implementation section can declare constants, types (including classes), variables, procedures, and functions that are private to the unit—that is, inaccessible to clients.

The implementation section can include its own uses clause, which must appear immediately after the word implementation. For information about the uses clause, see Unit references and the uses clause.


The initialization section
  Object Pascal Language Guide

The initialization section

Topic groups	See also

The initialization section is optional. It begins with the reserved word initialization and continues until the beginning of the finalization section or, if there is no finalization section, until the end of the unit. The initialization section contains statements that are executed, in the order in which they appear, on program start-up. So, for example, if you have defined data structures that need to be initialized, you can do this in the initialization section.

The initialization sections of units used by a client are executed in the order in which the units appear in the client’s uses clause.


The finalization section
  Object Pascal Language Guide

The finalization section

Topic groups	See also

The finalization section is optional and can appear only in units that have an initialization section. The finalization section begins with the reserved word finalization and continues until the end of the unit. It contains statements that are executed when the main program terminates. Use the finalization section to free resources that are allocated in the initialization section.

Finalization sections are executed in the opposite order from initializations. For example, if your application initializes units A, B, and C, in that order, it will finalize them in the order C, B, and A.

Once a unit’s initialization code starts to execute, the corresponding finalization section is guaranteed to execute when the application shuts down. The finalization section must therefore be able to handle incompletely initialized data, since, if a runtime error occurs, the initialization code might not execute completely.


Unit references and the uses clause
  Object Pascal Language Guide

Unit references and the uses clause

Topic groups	See also

A uses clause lists units used by the program, library, or unit in which the clause appears. (For information about libraries, see libraries and packages.) A uses clause can occur in

the project file for a program or library,

the interface section of a unit, and

the implementation section of a unit.

Most project files contain a uses clause, as do the interface sections of most units. The implementation section of a unit can contain its own uses clause as well.

The System unit is used automatically by every application and cannot be listed explicitly in the uses clause. (System implements routines for file I/O, string handling, floating point operations, dynamic memory allocation, and so forth.) Other standard library units, such as SysUtils, must be included in the uses clause. In most cases, all necessary units are placed in the uses clause when your project generates and maintains a source file.

For more information about the placement and content of the uses clause, see Multiple and indirect unit references and Circular unit references.

Uses clause syntax


The syntax of a uses clause
  Object Pascal Language Guide

The syntax of a uses clause

Topic groups	See also

A uses clause consists of the reserved word uses, followed by one or more comma-delimited unit names, followed by a semicolon. Examples:

uses Forms, Main;

uses Windows, Messages, SysUtils, Strings, Classes, Unit2, MyUnit;

uses SysUtils, Types, Classes, QGraphics, QControls, QForms, QDialogs;

In the uses clause of a program or library, any unit name may be followed by the reserved word in and the name of a source file, with or without a directory path, in single quotation marks; directory paths can be absolute or relative. Examples:

uses Windows, Messages, SysUtils, Strings in 'C:\Classes\Strings.pas', Classes;

uses

  QForms,

  Main,

  Extra in '../extra/extra.pas';

Include in ... after a unit name when you need to specify the unit’s source file. Since the IDE expects unit names to match the names of the source files in which they reside, there is usually no reason to do this. Using in is necessary only when the location of the source file is unclear, for example when

You have used a source file that is in a different directory from the project file, and that directory is not in the compiler’s search path or the general Library search path.

Different directories in the compiler’s search path have identically named units.

You are compiling a console application from the command line, and you have named a unit with an identifier that doesn’t match the name of its source file.

The compiler also relies on the in ... construction to determine which units are part of a project. Only units that appear in a project (.dpr) file’s uses clause followed by in and a file name are considered to be part of the project; other units in the uses clause are used by the project without belonging to it. This distinction has no effect on compilation, but it affects IDE tools like the Project Manager and Project Browser.

In the uses clause of a unit, you cannot use in to tell the compiler where to find a source file. Every unit must be in the compiler’s search path, the general Library search path, or the same directory as the unit that uses it. Moreover, unit names must match the names of their source files.


Multiple and indirect unit references
  Object Pascal Language Guide

Multiple and indirect unit references

Topic groups	See also

The order in which units appear in the uses clause determines the order of their initialization (see The initialization section)and affects the way identifiers are located by the compiler. If two units declare a variable, constant, type, procedure, or function with the same name, the compiler uses the one from the unit listed last in the uses clause. (To access the identifier from the other unit, you would have to add a qualifier: UnitName.Identifier.)

A uses clause need include only units used directly by the program or unit in which the clause appears. That is, if unit A references constants, types, variables, procedures, or functions that are declared in unit B, then A must use B explicitly. If B in turn references identifiers from unit C, then A is indirectly dependent on C; in this case, C needn’t be included in a uses clause in A, but the compiler must still be able to find both B and C in order to process A.

The example below illustrates indirect dependency.

program Prog;

uses Unit2;

const a = b;

...

unit Unit2;

interface

uses Unit1;

const b = c;

...

unit Unit1;

interface

const c = 1;

...

In this example, Prog depends directly on Unit2, which depends directly on Unit1. Hence Prog is indirectly dependent on Unit1. Because Unit1 does not appear in Prog’s uses clause, identifiers declared in Unit1 are not available to Prog.

To compile a client module, the compiler needs to locate all units that the client depends on, directly or indirectly. Unless the source code for these units has changed, however, the compiler needs only their .dcu (Windows) or .dcu/.dpu (Linux) files, not their source (.pas) files.

When changes are made in the interface section of a unit, other units that depend on it must be recompiled. But when changes are made only in the implementation or other sections of a unit, dependent units don’t have to be recompiled. The compiler tracks these dependencies automatically and recompiles units only when necessary.


Circular unit references
  Object Pascal Language Guide

Circular unit references

Topic groups	See also

When units reference each other directly or indirectly, the units are said to be mutually dependent. Mutual dependencies are allowed as long as there are no circular paths connecting the uses clause of one interface section to the uses clause of another. In other words, starting from the interface section of a unit, it must never be possible to return to that unit by following references through interface sections of other units. For a pattern of mutual dependencies to be valid, each circular reference path must lead through the uses clause of at least one implementation section.

In the simplest case of two mutually dependent units, this means that the units cannot list each other in their interface uses clauses. So the following example leads to a compilation error:

unit Unit1;

interface

uses Unit2;

...

unit Unit2;

interface

uses Unit1;

...

However, the two units can legally reference each other if one of the references is moved to the implementation section:

unit Unit1;

interface

uses Unit2;

...

unit Unit2;

interface

...

implementation

uses Unit1;

...

To reduce the chance of circular references, it’s a good idea to list units in the implementation uses clause whenever possible. Only when identifiers from another unit are used in the interface section is it necessary to list that unit in the interface uses clause.


Syntactic elements: Overview
  Object Pascal Language Guide

Syntactic elements

Topic groups	See also

Object Pascal uses the ASCII character set, including the letters A through Z and a through z, the digits 0 through 9, and other standard characters. It is not case-sensitive. The space character (ASCII 32) and the control characters (ASCII 0 through 31—including ASCII 13, the return or end-of-line character) are called blanks.

Fundamental syntactic elements, called tokens, combine to form expressions, declarations, and statements. A statement describes an algorithmic action that can be executed within a program. An expression is a syntactic unit that occurs within a statement and denotes a value. A declaration defines an identifier (such as the name of a function or variable) that can be used in expressions and statements, and, where appropriate, allocates memory for the identifier.


Fundamental syntactic elements: Overview
  Object Pascal Language Guide

Fundamental syntactic elements

Topic groups	See also

On the simplest level, a program is a sequence of tokens delimited by separators. A token is the smallest meaningful unit of text in a program. A separator is either a blank or a comment. Strictly speaking, it is not always necessary to place a separator between two tokens; for example, the code fragment

Size:=20;Price:=10;

is perfectly legal. Convention and readability, however, dictate that we write this as

Size := 20;

Price := 10;

Tokens are categorized as special symbols, identifiers, reserved words, directives, numerals, labels, and character strings. A separator can be part of a token only if the token is a character string. Adjacent identifiers, reserved words, numerals, and labels must have one or more separators between them.


Special symbols
  Object Pascal Language Guide

Special symbols

Topic groups	See also

Special symbols are nonalphanumeric characters, or pairs of such characters, that have fixed meanings. The following single characters are special symbols.

#   $   &   '   (   )   *   +   ,   –   .   /   :   ;   <   =   >   @   [   ]   ^   {   }

The following character pairs are also special symbols.

(*   (.   *)   .)   ..   //   :=   <=   >=   < >

The left bracket — [ — is equivalent to the character pair of left parenthesis and period — (. ; the right bracket — ] — is equivalent to the character pair of period and right parenthesis — .) . The left-parenthesis–plus–asterisk and asterisk–plus–right-parenthesis — (*  *) — are equivalent to the left and right brace — {  } .

Notice that !, " (double quotation marks), %, ?, \, _ (underscore), | (pipe), and ~ (tilde) are not special characters.


Identifiers
  Object Pascal Language Guide

Identifiers

Topic groups	See also

Identifiers denote constants, variables, fields, types, properties, procedures, functions, programs, units, libraries, and packages. An identifier can be of any length, but only the first 255 characters are significant. An identifier must begin with a letter or an underscore (_) and cannot contain spaces; letters, digits, and underscores are allowed after the first character. Reserved words cannot be used as identifiers.

Since Object Pascal is case-insensitive, an identifier like CalculateValue could be written in any of these ways:

CalculateValue

calculateValue

calculatevalue

CALCULATEVALUE

On Linux, the only identifiers for which case is important are unit names. Since unit names correspond to file names, inconsistencies in case can sometimes affect compilation.

Qualified identifiers

When you use an identifier that has been declared in more than one place, it is sometimes necessary to qualify the identifier. The syntax for a qualified identifier is

identifier1.identifier2

where identifier1 qualifies identifier2. For example, if two units each declare a variable called CurrentValue, you can specify that you want to access the CurrentValue in Unit2 by writing

Unit2.CurrentValue

Qualifiers can be iterated. For example,

Form1.Button1.Click

calls the Click method in Button1 of Form1.

If you don’t qualify an identifier, its interpretation is determined by the rules of scope described in Blocks and scope.


Reserved words
  Object Pascal Language Guide

Reserved words

Topic groups	See also

The following reserved words cannot be redefined or used as identifiers.

and	array	as	asm
begin	case	class	const
constructor	destructor	dispinterface	div
do	downto	else	end
except	exports	file	finalization
finally	for	function	goto
if	implementation	in	inherited
initialization	inline	interface	is
label	library	mod	nil
not	object	of	or
out	packed	procedure	program
property	raise	record	repeat
resourcestring	set	shl	shr
string	then	threadvar	to
try	type	unit	until
uses	var	while	with
xor
In addition to the words above, private, protected, public, published, and automated act as reserved words within object type declarations, but are otherwise treated as directives. The words at and on also have special meanings.


Directives
  Object Pascal Language Guide

Directives

Topic groups	See also

Directives are words that are sensitive in specific locations within source code. Directives have special meanings in Object Pascal, but, unlike reserved words, appear only in contexts where user-defined identifiers cannot occur. Hence—although it is inadvisable to do so—you can define an identifier that looks exactly like a directive.


Numerals
  Object Pascal Language Guide

Numerals

Topic groups	See also

Integer and real constants can be represented in decimal notation as sequences of digits without commas or spaces, and prefixed with the + or – operator to indicate sign. Values default to positive (so that, for example, 67258 is equivalent to +67258) and must be within the range of the largest predefined real or integer type.

Numerals with decimal points or exponents denote reals, while other numerals denote integers. When the character E or e occurs within a real, it means “times ten to the power of”. For example, 7E–2 means 7 [image] 10^–2, and 12.25e+6 and 12.25e6 both mean 12.25 [image] 10^6.

The dollar-sign prefix indicates a hexadecimal numeral—for example, $8F. Hexadecimal numbers without a preceding '-' unary operator are taken to be positive values. During an assignment, if a hexadecimal value lies outside the range of the receiving type an error is raised, except in the case of the Integer (32-bit Integer) where a warning is raised. In this case, values exceeding the positive range for Integer are taken to be negative numbers in a manner consistent with 2's complement integer representation.

For more information about real and integer types, see Data types, variables, and constants. For information about the data types of numerals, see True constants.


Labels
  Object Pascal Language Guide

Labels

Topic groups	See also

A label is a sequence of no more than four digits—that is, a numeral between 0 and 9999. Leading zeros are not significant. Identifiers can also function as labels.

Labels are used in goto statements. For more information about goto statements and labels, see Goto statements.


Character strings
  Object Pascal Language Guide

Character strings

Topic groups	See also

A character string, also called a string literal or string constant, consists of a quoted string, a control string, or a combination of quoted and control strings. Separators can occur only within quoted strings.

A quoted string is a sequence of up to 255 characters from the extended ASCII character set, written on one line and enclosed by apostrophes. A quoted string with nothing between the apostrophes is a null string. Two sequential apostrophes in a quoted string denote a single character, namely an apostrophe. For example,

'BORLAND'           { BORLAND }

'You''ll see'       { You'll see }

''''                { ' }

''                  { null string }

' '                 { a space }

A control string is a sequence of one or more control characters, each of which consists of the # symbol followed by an unsigned integer constant from 0 to 255 (decimal or hexadecimal) and denotes the corresponding ASCII character. The control string

#89#111#117

is equivalent to the quoted string

'You'

You can combine quoted strings with control strings to form larger character strings. For example, you could use

'Line 1'#13#10'Line 2'

to put a carriage-return–line-feed between “Line 1” and “Line 2”. However, you cannot concatenate two quoted strings in this way, since a pair of sequential apostrophes is interpreted as a single character. (To concatenate quoted strings, use the + operator or simply combine them into a single quoted string.)

A character string’s length is the number of characters in the string. A character string of any length is compatible with any string type and with the PChar type. A character string of length 1 is compatible with any character type, and, when extended syntax is enabled ({$X+}), a nonempty character string of length n is compatible with zero-based arrays and packed arrays of n characters. For more information about string types, see String types.


Comments and compiler directives
  Object Pascal Language Guide

Comments and compiler directives

Topic groups	See also

Comments are ignored by the compiler, except when they function as separators (delimiting adjacent tokens) or compiler directives.

There are several ways to construct comments:

{ Text between a left brace and a right brace constitutes a comment. }

(* Text between a left-parenthesis-plus-asterisk and an

   asterisk-plus-right-parenthesis also constitutes a comment. *)

// Any text between a double-slash and the end of the line constitutes a comment.

A comment that contains a dollar sign ($) immediately after the opening { or (* is a compiler directive. For example,

{$WARNINGS OFF}

tells the compiler not to generate warning messages.


About expressions
  Object Pascal Language Guide

Expressions

Topic groups	See also

An expression is a construction that returns a value. For example,

X                   { variable }

@X                  { address of a variable }

15                  { integer constant }

InterestRate        { variable }

Calc(X,Y)           { function call }

X * Y               { product of X and Y }

Z / (1 - Z)         { quotient of Z and (1 - Z) }

X = 1.5             { Boolean }

C in Range1         { Boolean }

not Done            { negation of a Boolean }

['a','b','c']       { set }

Char(48)            { value typecast }

The simplest expressions are variables and constants (described in Data types, variables, and constants). More complex expressions are built from simpler ones using operators, function calls, set constructors, indexes, and typecasts.


About operators
  Object Pascal Language Guide

Operators

Topic groups	See also

Operators behave like predefined functions that are part of the Object Pascal language. For example, the expression (X + Y) is built from the variables X and Y—called operands—with the + operator; when X and Y represent integers or reals, (X + Y) returns their sum. Operators include @, not, ^, *, /, div, mod, and, shl, shr, as, +, –, or, xor, =, >, <, <>, <=, >=, in, and is.

The operators @, not, and ^ are unary (taking one operand). All other operators are binary (taking two operands), except that + and – can function as either unary or binary. A unary operator always precedes its operand (for example, -B), except for ^, which follows its operand (for example, P^). A binary operator is placed between its operands (for example, A = 7).

Some operators behave differently depending on the type of data passed to them. For example, not performs bitwise negation on an integer operand and logical negation on a Boolean operand. Such operators appear below under multiple categories.

Except for ^, is, and in, all operators can take operands of type Variant.

The sections that follow assume some familiarity with Object Pascal data types.

For information about operator precedence in complex expressions, see Operator precedence rules.

Arithmetic operators

Boolean operators

Logical (bitwise) operators

String operators

Pointer operators

Set operators

Relational operators

Class operators

The @ operator


Arithmetic operators
  Object Pascal Language Guide

Arithmetic operators

Topic groups	See also

Arithmetic operators, which take real or integer operands, include +, –, *, /, div, and mod.

Operator	Operation	Operand types	Result type	Example
+	addition	integer, real	integer, real	X + Y
–	subtraction	integer, real	integer, real	Result - 1
*	multiplication	integer, real	integer, real	P * InterestRate
/	real division	integer, real	real	X / 2
div	integer division	integer	integer	Total div UnitSize
mod	remainder	integer	integer	Y mod 6
Operator	Operation	Operand type	Result type	Example
+ (unary)	sign identity	integer, real	integer, real	+7
– (unary)	sign negation	integer, real	integer, real	-X
The following rules apply to arithmetic operators.

The value of x/y is of type Extended, regardless of the types of x and y. For other arithmetic operators, the result is of type Extended whenever at least one operand is a real; otherwise, the result is of type Int64 when at least one operand is of type Int64; otherwise, the result is of type Integer. If an operand’s type is a subrange of an integer type, it is treated as if it were of the integer type.

The value of x div y is the value of x/y rounded in the direction of zero to the nearest integer.

The mod operator returns the remainder obtained by dividing its operands. In other words, x mod y = x – (x div y) * y.

A runtime error occurs when y is zero in an expression of the form x/y, x div y, or x mod y.


Boolean operators
  Object Pascal Language Guide

Boolean operators

Topic groups	See also

The Boolean operators not, and, or, and xor take operands of any Boolean type and return a value of type Boolean.

Operator	Operation	Operand types	Result type	Example
not	negation	Boolean	Boolean	not (C in MySet)
and	conjunction	Boolean	Boolean	Done and (Total > 0)
or	disjunction	Boolean	Boolean	A or B
xor	exclusive disjunction	Boolean	Boolean	A xor B
These operations are governed by standard rules of Boolean logic. For example, an expression of the form x and y is True if and only if both x and y are True.

Complete versus short-circuit Boolean evaluation

The compiler supports two modes of evaluation for the and and or operators: complete evaluation and short-circuit (partial) evaluation. Complete evaluation means that each conjunct or disjunct is evaluated, even when the result of the entire expression is already determined. Short-circuit evaluation means strict left-to-right evaluation that stops as soon as the result of the entire expression is determined. For example, if the expression A and B is evaluated under short-circuit mode when A is False, the compiler won’t evaluate B; it knows that the entire expression is False as soon as it evaluates A.

Short-circuit evaluation is usually preferable because it guarantees minimum execution time and, in most cases, minimum code size. Complete evaluation is sometimes convenient when one operand is a function with side effects that alter the execution of the program.

Short-circuit evaluation also allows the use of constructions that might otherwise result in illegal runtime operations. For example, the following code iterates through the string S, up to the first comma.

while (I <= Length(S)) and (S[I] <> ',') do

begin

  ...

  Inc(I);

end;

In a case where S has no commas, the last iteration increments I to a value which is greater than the length of S. When the while condition is next tested, complete evaluation results in an attempt to read S[I], which could cause a runtime error. Under short-circuit evaluation, in contrast, the second part of the while condition — (S[I] <> ',') — is not evaluated after the first part fails.

Use the $B compiler directive to control evaluation mode. The default state is {$B–}, which enables short-circuit evaluation. To enable complete evaluation locally, add the {$B+} directive to your code. You can also switch to complete evaluation on a project-wide basis by selecting Complete Boolean Evaluation in the Compiler Options dialog.

Note:	If either operand involves a variant, the compiler always performs complete evaluation (even in the {$B–} state).


Logical (bitwise) operators
  Object Pascal Language Guide

Logical (bitwise) operators

Topic groups	See also

The following logical operators perform bitwise manipulation on integer operands. For example, if the value stored in X (in binary) is 001101 and the value stored in Y is 100001, the statement

Z := X or Y;

assigns the value 101101 to Z.

Operator	Operation	Operand types	Result type	Examples
not	bitwise negation	integer	integer	not X
and	bitwise and	integer	integer	X and Y
or	bitwise or	integer	integer	X or Y
xor	bitwise xor	integer	integer	X xor Y
shl	bitwise shift left	integer	integer	X shl 2
shr	bitwise shift right	integer	integer	Y shr I
The following rules apply to bitwise operators.

The result of a not operation is of the same type as the operand.

If the operands of an and, or, or xor operation are both integers, the result is of the predefined integer type with the smallest range that includes all possible values of both types.

The operations x shl y and x shr y shift the value of x to the left or right by y bits, which is equivalent to multiplying or dividing x by 2^y; the result is of the same type as x. For example, if N stores the value 01101 (decimal 13), then N shl 1 returns 11010 (decimal 26). Note that the value of y is interpreted modulo the size of the type of x. Thus for example, if x is an integer, x shl 40 is interpreted as x shl 8 because an integer is 32 bits and 40 mod 32 is 8.


String operators
  Object Pascal Language Guide

String operators

Topic groups	See also

The relational operators =, <>, <, >, <=, and >= all take string operands (see Relational operators). The + operator concatenates two strings.

Operator	Operation	Operand types	Result type	Example
+	concatenation	string, packed string, character	string	S + '. '
The following rules apply to string concatenation.

The operands for + can be strings, packed strings (packed arrays of type Char), or characters. However, if one operand is of type WideChar, the other operand must be a long string.

The result of a + operation is compatible with any string type. However, if the operands are both short strings or characters, and their combined length is greater than 255, the result is truncated to the first 255 characters.


Pointer operators
  Object Pascal Language Guide

Pointer operators

Topic groups	See also

The relational operators <, >, <=, and >= can take operands of type PChar (see Relational operators). The following operators also take pointers as operands. For more information about pointers, see Pointers and pointer types.

Operator	Operation	Operand types	Result type	Example
+	pointer addition	character pointer, integer	character pointer	P + I
-	pointer subtraction	character pointer, integer	character pointer, integer	P - Q
^	pointer dereference	pointer	base type of pointer	P^
=	equality	pointer	Boolean	P = Q
<>	inequality	pointer	Boolean	P <> Q
The ^ operator dereferences a pointer. Its operand can be a pointer of any type except the generic Pointer, which must be typecast before dereferencing.

P = Q is True just in case P and Q point to the same address; otherwise, P <> Q is True.

You can use the + and – operators to increment and decrement the offset of a character pointer. You can also use – to calculate the difference between the offsets of two character pointers. The following rules apply.

If I is an integer and P is a character pointer, then P + I adds I to the address given by P; that is, it returns a pointer to the address I characters after P. (The expression I + P is equivalent to P + I.) P – I subtracts I from the address given by P; that is, it returns a pointer to the address I characters before P.

If P and Q are both character pointers, then P – Q computes the difference between the address given by P (the higher address) and the address given by Q (the lower address); that is, it returns an integer denoting the number of characters between P and Q. P + Q is not defined.


Set operators
  Object Pascal Language Guide

Set operators

Topic groups	See also

The following operators take sets as operands.

Operator	Operation	Operand types	Result type	Example
+	union	set	set	Set1 + Set2
–	difference	set	set	S - T
*	intersection	set	set	S * T
<=	subset	set	Boolean	Q <= MySet
>=	superset	set	Boolean	S1 >= S2
=	equality	set	Boolean	S2 = MySet
<>	inequality	set	Boolean	MySet <> S1
in	membership	ordinal, set	Boolean	A in Set1
The following rules apply to +, –, and *.

An ordinal O is in X + Y if and only if O is in X or Y (or both). O is in X – Y if and only if O is in X but not in Y. O is in X * Y if and only if O is in both X and Y.

The result of a +, –, or * operation is of the type set of A..B, where A is the smallest ordinal value in the result set and B is the largest.

The following rules apply to <=, >=, =, <>, and in.

X <= Y is True just in case every member of X is a member of Y; Z >= W is equivalent to W <= Z. U = V is True just in case U and V contain exactly the same members; otherwise, U <> V is True.

For an ordinal O and a set S, O in S is True just in case O is a member of S.


Relational operators
  Object Pascal Language Guide

Relational operators

Topic groups	See also

Relational operators are used to compare two operands. The operators =, <>, <=, and >= also apply to sets (see Set operators); = and <> also apply to pointers (see Pointer operators).

Operator	Operation	Operand types	Result type	Example
=	equality	simple, class, class reference, interface, string, packed string	Boolean	I = Max
<>	inequality	simple, class, class reference, interface, string, packed string	Boolean	X <> Y
<	less-than	simple, string, packed string, PChar	Boolean	X < Y
>	greater-than	simple, string, packed string, PChar	Boolean	Len > 0
<=	less-than-or-equal-to	simple, string, packed string, PChar	Boolean	Cnt <= I
>=	greater-than-or-equal-to	simple, string, packed string, PChar	Boolean	I >= 1
For most simple types, comparison is straightforward. For example, I = J is True just in case I and J have the same value, and I <> J is True otherwise. The following rules apply to relational operators.

Operands must be of compatible types, except that a real and an integer can be compared.

Strings are compared according to the ordering of the extended ASCII character set. Character types are treated as strings of length 1.

Two packed strings must have the same number of components to be compared. When a packed string with n components is compared to a string, the packed string is treated as a string of length n.

The operators <, >, <=, and >= apply to PChar operands only if the two pointers point within the same character array.

The operators = and <> can take operands of class and class-reference types. With operands of a class type, = and <> are evaluated according the rules that apply to pointers: C = D is True just in case C and D point to the same instance object, and C <> D is True otherwise. With operands of a class-reference type, C = D is True just in case C and D denote the same class, and C <> D is True otherwise. For more information about classes, see Classes and objects.


Class operators
  Object Pascal Language Guide

Class operators

Topic groups	See also

The operators as and is take classes and instance objects as operands; as operates on interfaces as well. For more information, see Classes and objects and Object interfaces.

The relational operators = and <> also operate on classes. See Relational operators.


The @ operator
  Object Pascal Language Guide

The @ operator

Topic groups	See also

The @ operator returns the address of a variable, or of a function, procedure, or method; that is, @ constructs a pointer to its operand. For more information about pointers, see Pointers and pointer types. The following rules apply to @.

If X is a variable, @X returns the address of X. (Special rules apply when X is a procedural variable; see Procedural types in statements and expressions.) The type of @X is Pointer if the default {$T–} compiler directive is in effect. In the {$T+} state, @X is of type ^T, where T is the type of X.

If F is a routine (a function or procedure), @F returns F’s entry point. The type of @F is always Pointer.

When @ is applied to a method defined in a class, the method identifier must be qualified with the class name. For example,

@TMyClass.DoSomething

points to the DoSomething method of TMyClass. For more information about classes and methods, see Classes and objects.


Operator precedence rules
  Object Pascal Language Guide

Operator precedence rules

Topic groups	See also

In complex expressions, rules of precedence determine the order in which operations are performed.

Operators	Precedence
@, not	first (highest)
*, /, div, mod, and, shl, shr, as	second
+, –, or, xor	third
=, <>, <, >, <=, >=, in, is	fourth (lowest)
An operator with higher precedence is evaluated before an operator with lower precedence, while operators of equal precedence associate to the left. Hence the expression

X + Y * Z

multiplies Y times Z, then adds X to the result; * is performed first, because is has a higher precedence than +. But

X - Y + Z

first subtracts Y from X, then adds Z to the result; – and + have the same precedence, so the operation on the left is performed first.

You can use parentheses to override these precedence rules. An expression within parentheses is evaluated first, then treated as a single operand. For example,

(X + Y) * Z

multiplies Z times the sum of X and Y.

Parentheses are sometimes needed in situations where, at first glance, they seem not to be. For example, consider the expression

X = Y or X = Z

The intended interpretation of this is obviously

(X = Y) or (X = Z)

Without parentheses, however, the compiler follows operator precedence rules and reads it as

(X = (Y or X)) = Z

—which results in a compilation error unless Z is Boolean.

Parentheses often make code easier to write and to read, even when they are, strictly speaking, superfluous. Thus the first example above could be written as

X + (Y * Z)

Here the parentheses are unnecessary (to the compiler), but they spare both programmer and reader from having to think about operator precedence.


Function calls
  Object Pascal Language Guide

Function calls

Topic groups	See also

Because functions return a value, function calls are expressions. For example, if you’ve defined a function called Calc that takes two integer arguments and returns an integer, then the function call Calc(24, 47) is an integer expression. If I and J are integer variables, then I + Calc(J, 8) is also an integer expression. Examples of function calls include

Sum(A, 63)

Maximum(147, J)

Sin(X + Y)

Eof(F)

Volume(Radius, Height)

GetValue

TSomeObject.SomeMethod(I,J);

For more information about functions, see Procedures and functions.


Set constructors
  Object Pascal Language Guide

Set constructors

Topic groups	See also

A set constructor denotes a set-type value. For example,

[5, 6, 7, 8]

denotes the set whose members are 5, 6, 7, and 8. The set constructor

[ 5..8 ]

could also denote the same set.

The syntax for a set constructor is

[ item1, ..., itemn ]

where each item is either an expression denoting an ordinal of the set’s base type or a pair of such expressions with two dots (..) in between. When an item has the form x..y, it is shorthand for all the ordinals in the range from x to y, inclusive; but if x is greater than y, then x..y denotes nothing and [x..y] is the empty set. The set constructor [ ] denotes the empty set, while [x] denotes the set whose only member is the value of x.

Examples of set constructors:

[red, green, MyColor]

[1, 5, 10..K mod 12, 23]

['A'..'Z', 'a'..'z', Chr(Digit + 48)]

For more information about sets, see Sets.


Indexes
  Object Pascal Language Guide

Indexes

Topic groups	See also

Strings, arrays, array properties, and pointers to strings or arrays can be indexed. For example, if FileName is a string variable, the expression FileName[3] returns the third character in the string denoted by FileName, while FileName[I + 1] returns the character immediately after the one indexed by I. For information about strings, see String types. For information about arrays and array properties, see Arrays and Array properties.


Typecasts: Overview
  Object Pascal Language Guide

Typecasts

Topic groups	See also

It is sometimes useful to treat an expression as if it belonged to different type. A typecast allows you to do this by, in effect, temporarily changing an expression’s type. For example, Integer('A') casts the character A as an integer.

The syntax for a typecast is

typeIdentifier(expression)

If the expression is a variable, the result is called a variable typecast; otherwise, the result is a value typecast. While their syntax is the same, different rules apply to the two kinds of typecast.

Value typecasts

Variable typecasts


Value typecasts
  Object Pascal Language Guide

Value typecasts

Topic groups	See also

In a value typecast, the type identifier and the cast expression must both be ordinal types or both be pointer types. Examples of value typecasts include

Integer('A')

Char(48)

Boolean(0)

Color(2)

Longint(@Buffer)

The resulting value is obtained by converting the expression in parentheses. This may involve truncation or extension if the size of the specified type differs from that of the expression. The expression’s sign is always preserved.

The statement

I := Integer('A');

assigns the value of Integer('A')—that is, 65—to the variable I.

A value typecast cannot be followed by qualifiers and cannot appear on the left side of an assignment statement.


Variable typecasts
  Object Pascal Language Guide

Variable typecasts

Topic groups	See also

You can cast any variable to any type, provided their sizes are the same and you do not mix integers with reals. (To convert numeric types, rely on standard functions like Int and Trunc.) Examples of variable typecasts include

Char(I)

Boolean(Count)

TSomeDefinedType(MyVariable)

Variable typecasts can appear on either side of an assignment statement. Thus

var MyChar: char;

...

Shortint(MyChar) := 122;

assigns the character z (ASCII 122) to MyChar.

You can cast variables to a procedural type. For example, given the declarations

type Func = function(X: Integer): Integer;

var

  F: Func;

  P: Pointer;

  N: Integer;

you can make the following assignments.

F := Func(P);           { Assign procedural value in P to F }

Func(P) := F;           { Assign procedural value in F to P }

@F := P;                { Assign pointer value in P to F }

P := @F;                { Assign pointer value in F to P }

N := F(N);              { Call function via F }

N := Func(P)(N);        { Call function via P }

Variable typecasts can also be followed by qualifiers, as illustrated in the following example.

type

  TByteRec = record

    Lo, Hi: Byte;

  end;

  TWordRec = record

    Low, High: Word;

  end;

  PByte = ^Byte;

var

  B: Byte;

  W: Word;

  L: Longint;

  P: Pointer;

begin

  W := $1234;

  B := TByteRec(W).Lo;

  TByteRec(W).Hi := 0;

  L := $01234567;

  W := TWordRec(L).Low;

  B := TByteRec(TWordRec(L).Low).Hi;

  B := PByte(L)^;

end;

In this example, TByteRec is used to access the low- and high-order bytes of a word, and TWordRec to access the low- and high-order words of a long integer. You could call the predefined functions Lo and Hi for the same purpose, but a variable typecast has the advantage that it can be used on the left side of an assignment statement.

For information about typecasting pointers, see Pointers and pointer types. For information about casting class and interface types, see The as operator and Interface typecasts.


About declarations and statements
  Object Pascal Language Guide

Declarations and statements

Topic groups	See also

Aside from the uses clause (and reserved words like implementation that demarcate parts of a unit), a program consists entirely of declarations and statements, which are organized into blocks.

Declarations

Statements


Declarations
  Object Pascal Language Guide

Declarations

Topic groups	See also

The names of variables, constants, types, fields, properties, procedures, functions, programs, units, libraries, and packages are called identifiers. (Numeric constants like 26057 are not identifiers.) Identifiers must be declared before you can use them; the only exceptions are a few predefined types, routines, and constants that the compiler understands automatically, the variable Result when it occurs inside a function block, and the variable Self when it occurs inside a method implementation.

A declaration defines an identifier and, where appropriate, allocates memory for it. For example,

var Size: Extended;

declares a variable called Size that holds an Extended (real) value, while

function DoThis(X, Y: string): Integer;

declares a function called DoThis that takes two strings as arguments and returns an integer. Each declaration ends with a semicolon. When you declare several variables, constants, types, or labels at the same time, you need only write the appropriate reserved word once:

var

  Size: Extended;

  Quantity: Integer;

  Description: string;

The syntax and placement of a declaration depend on the kind of identifier you are defining. In general, declarations can occur only at the beginning of a block or at the beginning of the interface or implementation section of a unit (after the uses clause). Specific conventions for declaring variables, constants, types, functions, and so forth are explained in the documentation for those topics.

The “hint” directives platform, deprecated, and library may be appended to any declaration. In the case of a procedure or function declaration, the hint directive should be separated from the rest of the declaration with a semicolon. Examples:

procedure SomeOldRoutine; stdcall; deprecated;

var VersionNumber: Real library;

type AppError = class(Exception)

 ...

end platform;

When source code is compiled in the {$HINTS ON} {$WARNINGS ON} state, each reference to an identifier declared with one of these directives generates an appropriate hint or warning. Use platform to mark items that are specific to a particular operating environment (such as Windows or Linux), deprecated to indicate that an item is obsolete or supported only for backward compatibility, and library to flag dependencies on a particular library or component framework (such as VCL or CLX).


Statements
  Object Pascal Language Guide

Statements

Topic groups	See also

Statements define algorithmic actions within a program. Simple statements—like assignments and procedure calls—can combine to form loops, conditional statements, and other structured statements.

Multiple statements within a block, and in the initialization or finalization section of a unit, are separated by semicolons.

Simple statements

Structured statements


Simple statements: Overview
  Object Pascal Language Guide

Simple statements

Topic groups	See also

A simple statement doesn’t contain any other statements. Simple statements include assignments, calls to procedures and functions, and goto jumps.


Assignment statements
  Object Pascal Language Guide

Assignment statements

Topic groups	See also

An assignment statement has the form

variable := expression

where variable is any variable reference—including a variable, variable typecast, dereferenced pointer, or component of a structured variable—and expression is any assignment-compatible expression. (Within a function block, variable can be replaced with the name of the function being defined. See Procedures and functions.) The := symbol is sometimes called the assignment operator.

An assignment statement replaces the current value of variable with the value of expression. For example,

I := 3;

assigns the value 3 to the variable I. The variable reference on the left side of the assignment can appear in the expression on the right. For example,

I := I + 1;

increments the value of I. Other assignment statements include

X := Y + Z;

Done := (I >= 1) and (I < 100);

Hue1 := [Blue, Succ(C)];

I := Sqr(J) - I  * K;

Shortint(MyChar) := 122;

TByteRec(W).Hi := 0;

MyString[I] := 'A';

SomeArray[I + 1] := P^;

TMyObject.SomeProperty := True;


Procedure and function calls
  Object Pascal Language Guide

Procedure and function calls

Topic groups	See also

A procedure call consists of the name of a procedure (with or without qualifiers), followed by a parameter list (if required). Examples include

PrintHeading;

Transpose(A, N, M);

Find(Smith, William);

Writeln('Hello world!');

DoSomething();

Unit1.SomeProcedure;

TMyObject.SomeMethod(X,Y);

With extended syntax enabled ({$X+}), function calls, like calls to procedures, can be treated as statements in their own right:

MyFunction(X);

When you use a function call in this way, its return value is discarded.

For more information about procedures and functions, see Procedures and functions.


Goto statements
  Object Pascal Language Guide

Goto statements

Topic groups	See also

A goto statement, which has the form

goto label

transfers program execution to the statement marked by the specified label. To mark a statement, you must first declare the label. Then precede the statement you want to mark with the label and a colon:

label: statement

Declare labels like this:

label label;

You can declare several labels at once:

label label1, ..., labeln;

A label can be any valid identifier or any numeral between 0 and 9999.

The label declaration, marked statement, and goto statement must belong to the same block. (See Blocks and scope.) Hence it is not possible to jump into or out of a procedure or function. Do not mark more than one statement in a block with the same label.

For example,

label StartHere;

...

StartHere: Beep;

goto StartHere;

creates an infinite loop that calls the Beep procedure repeatedly.

The goto statement is generally discouraged in structured programming. It is, however, sometimes used as a way of exiting from nested loops, as in the following example.

procedure FindFirstAnswer;

var X, Y, Z, Count: Integer;

label FoundAnAnswer;

begin

  Count := SomeConstant;

  for X := 1 to Count do

    for Y := 1 to Count do

      for Z := 1 to Count do

        if ... { some condition holds on X, Y, and Z } then

          goto FoundAnAnswer;

  ... {code to execute if no answer is found }

  Exit;

  FoundAnAnswer:

    ... { code to execute when an answer is found }

end;

Notice that we are using goto to jump out of a nested loop. Never jump into a loop or other structured statement, since this can have unpredictable effects.


Structured statements: Overview
  Object Pascal Language Guide

Structured statements

Topic groups	See also

Structured statements are built from other statements. Use a structured statement when you want to execute other statements sequentially, conditionally, or repeatedly.

A compound or with statement simply executes a sequence of constituent statements.

A conditional statement—that is, an if or case statement—executes at most one of its constituents, depending on specified criteria.

Loop statements—including repeat, while, and for loops—execute a sequence of constituent statements repeatedly.

A special group of statements—including raise, try...except, and try...finally constructions—create and handle exceptions. For information about exception generation and handling, see Exceptions.


Compound statements
  Object Pascal Language Guide

Compound statements

Topic groups	See also

A compound statement is a sequence of other (simple or structured) statements to be executed in the order in which they are written. The compound statement is bracketed by the reserved words begin and end, and its constituent statements are separated by semicolons. For example:

begin

  Z := X;

  X := Y;

  Y := Z;

end;

The last semicolon before end is optional. So we could have written this as

begin

  Z := X;

  X := Y;

  Y := Z

end;

Compound statements are essential in contexts where Object Pascal syntax requires a single statement. In addition to program, function, and procedure blocks, they occur within other structured statements, such as conditionals or loops. For example:

begin

  I := SomeConstant;

  while I > 0 do

  begin

    ...

    I := I - 1;

  end;

end;

You can write a compound statement that contains only a single constituent statement; like parentheses in a complex term, begin and end sometimes serve to disambiguate and to improve readability. You can also use an empty compound statement to create a block that does nothing:

begin

end;


With statements
  Object Pascal Language Guide

With statements

Topic groups	See also

A with statement is a shorthand for referencing the fields of a record or the fields, properties, and methods of an object. The syntax of a with statement is

with obj do statement

or

with obj1, ..., objn do statement

where obj is a variable reference denoting an object or record, and statement is any simple or structured statement. Within statement, you can refer to fields, properties, and methods of obj using their identifiers alone—without qualifiers.

For example, given the declarations

type TDate = record

  Day: Integer;

  Month: Integer;

  Year: Integer;

end;

var OrderDate: TDate;

you could write the following with statement.

with OrderDate do

  if Month = 12 then

  begin

    Month := 1;

    Year := Year + 1;

  end

  else

    Month := Month + 1;

This is equivalent to

if OrderDate.Month = 12 then

begin

  OrderDate.Month := 1;

  OrderDate.Year := OrderDate.Year + 1;

end

else

  OrderDate.Month := OrderDate.Month + 1;

If the interpretation of obj involves indexing arrays or dereferencing pointers, these actions are performed once, before statement is executed. This makes with statements efficient as well as concise. It also means that assignments to a variable within statement cannot affect the interpretation of obj during the current execution of the with statement.

Each variable reference or method name in a with statement is interpreted, if possible, as a member of the specified object or record. If there is another variable or method of the same name that you want to access from the with statement, you need to prepend it with a qualifier, as in the following example.

with OrderDate do

  begin

    Year := Unit1.Year

    ...

  end;

When multiple objects or records appear after with, the entire statement is treated like a series of nested with statements. Thus

with obj1, obj2, ..., objn do statement

is equivalent to

with obj1 do

  with obj2 do

     ...

     with objn do

       statement

In this case, each variable reference or method name in statement is interpreted, if possible, as a member of objn; otherwise it is interpreted, if possible, as a member of objn–1; and so forth. The same rule applies to interpreting the objs themselves, so that, for instance, if objn is a member of both obj1 and obj2, it is interpreted as obj2.objn.


If statements
  Object Pascal Language Guide

If statements

Topic groups	See also

There are two forms of if statement: if...then and the if...then...else. The syntax of an if...then statement is

if expression then statement

where expression returns a Boolean value. If expression is True, then statement is executed; otherwise it is not. For example,

if J <> 0 then Result := I/J;

The syntax of an if...then...else statement is

if expression then statement1 else statement2

where expression returns a Boolean value. If expression is True, then statement1 is executed; otherwise statement2 is executed. For example,

if J = 0 then

  Exit

else

  Result := I/J;

The then and else clauses contain one statement each, but it can be a structured statement. For example,

if J <> 0 then

begin

  Result := I/J;

  Count := Count + 1;

end

else if Count = Last then

  Done := True

else

  Exit;

Notice that there is never a semicolon between the then clause and the word else. You can place a semicolon after an entire if statement to separate it from the next statement in its block, but the then and else clauses require nothing more than a space or carriage return between them. Placing a semicolon immediately before else (in an if statement) is a common programming error.

A special difficulty arises in connection with nested if statements. The problem arises because some if statements have else clauses while others do not, but the syntax for the two kinds of statement is otherwise the same. In a series of nested conditionals where there are fewer else clauses than if statements, it may not seem clear which else clauses are bound to which ifs. Consider a statement of the form

if expression1 then if expression2 then statement1 else statement2;

There would appear to be two ways to parse this:

if expression1 then [ if expression2 then statement1 else statement2 ];

if expression1 then [ if expression2 then statement1 ] else statement2;

The compiler always parses in the first way. That is, in real code, the statement

if ... { expression1 } then

  if ... { expression2 } then

    ... { statement1 }

  else

    ... { statement2 } ;

is equivalent to

if ... { expression1 } then

begin

  if ... { expression2 } then

    ... { statement1 }

  else

    ... { statement2 }

end;

The rule is that nested conditionals are parsed starting from the innermost conditional, with each else bound to the nearest available if on its left. To force the compiler to read our example in the second way, you would have to write it explicitly as

if ... { expression1 } then

begin

  if ... { expression2 } then

    ... { statement1 }

end

else

  ... { statement2 } ;


Case statements
  Object Pascal Language Guide

Case statements

Topic groups	See also

The case statement provides a readable alternative to complex nested if conditionals. A case statement has the form

case selectorExpression of

  caseList1: statement1;

  ...

  caseListn: statementn;

end

where selectorExpression is any expression of an ordinal type (string types are invalid) and each caseList is one of the following:

A numeral, declared constant, or other expression that the compiler can evaluate without executing your program. It must be of an ordinal type compatible with selectorExpression. Thus 7, True, 4 + 5 * 3, 'A', and Integer('A') can all be used as caseLists, but variables and most function calls cannot. (A few built-in functions like Hi and Lo can occur in a caseList. See Constant expressions.)

A subrange having the form First..Last, where First and Last both satisfy the criterion above and First is less than or equal to Last.

A list having the form item1, ..., itemn, where each item satisfies one of the criteria above.

Each value represented by a caseList must be unique in the case statement; subranges and lists cannot overlap. A case statement can have a final else clause:

case selectorExpression of

  caseList1: statement1;

  ...

  caseListn: statementn;

else

  statements;

end

where statements is a semicolon-delimited sequence of statements. When a case statement is executed, at most one of statement1 ... statementn is executed. Whichever caseList has a value equal to that of selectorExpression determines the statement to be used. If none of the caseLists has the same value as selectorExpression, then the statements in the else clause (if there is one) are executed.

The case statement

case I of

  1..5: Caption := 'Low';

  6..9: Caption := 'High';

  0, 10..99: Caption := 'Out of range';

else

  Caption := '';

end;

is equivalent to the nested conditional

if I in [1..5] then

  Caption := 'Low'

  else if I in [6..10] then

    Caption := 'High'

    else if (I = 0) or (I in [10..99]) then

      Caption := 'Out of range'

      else

        Caption := '';

Other examples of case statements:

case MyColor of

  Red: X := 1;

  Green: X := 2;

  Blue: X := 3;

  Yellow, Orange, Black: X := 0;

end;

case Selection of

  Done: Form1.Close;

  Compute: CalculateTotal(UnitCost, Quantity);

else

  Beep;

end;


Control loops
  Object Pascal Language Guide

Control loops

Topic groups	See also

Loops allow you to execute a sequence of statements repeatedly, using a control condition or variable to determine when the execution stops. Object Pascal has three kinds of control loop: repeat statements, while statements, and for statements.

You can use the standard Break and Continue procedures to control the flow of a repeat, while, or for statement. Break terminates the statement in which it occurs, while Continue begins executing the next iteration of the sequence.

Repeat statements

While statements

For statements


Repeat statements
  Object Pascal Language Guide

Repeat statements

Topic groups	See also

The syntax of a repeat statement is

repeat statement1; ...; statementn; until expression

where expression returns a Boolean value. (The last semicolon before until is optional.) The repeat statement executes its sequence of constituent statements continually, testing expression after each iteration. When expression returns True, the repeat statement terminates. The sequence is always executed at least once because expression is not evaluated until after the first iteration.

Examples of repeat statements include

repeat

  K := I mod J;

  I := J;

  J := K;

until J = 0;

repeat

  Write('Enter a value (0..9): ');

  Readln(I);

until (I >= 0) and (I <= 9);


While statements
  Object Pascal Language Guide

While statements

Topic groups	See also

A while statement is similar to a repeat statement, except that the control condition is evaluated before the first execution of the statement sequence. Hence, if the condition is false, the statement sequence is never executed.

The syntax of a while statement is

while expression do statement

where expression returns a Boolean value and statement can be a compound statement. The while statement executes its constituent statement repeatedly, testing expression before each iteration. As long as expression returns True, execution continues.

Examples of while statements include

while Data[I] <> X do I := I + 1;

while I > 0 do

begin

  if Odd(I) then Z := Z * X;

  I := I div 2;

  X := Sqr(X);

end;

while not Eof(InputFile) do

begin

  Readln(InputFile, Line);

  Process(Line);

end;


For statements
  Object Pascal Language Guide

For statements

Topic groups	See also

A for statement, unlike a repeat or while statement, requires you to specify explicitly the number of iterations you want the loop to go through. The syntax of a for statement is

for counter := initialValue to finalValue do statement

or

for counter := initialValue downto finalValue do statement

where

counter is a local variable (declared in the block containing the for statement) of ordinal type, without any qualifiers.

initialValue and finalValue are expressions that are assignment-compatible with counter.

statement is a simple or structured statement that does not change the value of counter.

The for statement assigns the value of initialValue to counter, then executes statement repeatedly, incrementing or decrementing counter after each iteration. (The for...to syntax increments counter, while the for...downto syntax decrements it.) When counter returns the same value as finalValue, statement is executed once more and the for statement terminates. In other words, statement is executed once for every value in the range from initialValue to finalValue. If initialValue is equal to finalValue, statement is executed exactly once. If initialValue is greater than finalValue in a for...to statement, or less than finalValue in a for...downto statement, then statement is never executed. After the for statement terminates, the value of counter is undefined.

For purposes of controlling execution of the loop, the expressions initialValue and finalValue are evaluated only once, before the loop begins. Hence the for...to statement is almost, but not quite, equivalent to this while construction:

begin

  counter := initialValue;

  while counter <= finalValue do

  begin

    statement;

    counter := Succ(counter);

  end;

end

The difference between this construction and the for...to statement is that the while loop re-evaluates finalValue before each iteration. This can result in noticeably slower performance if finalValue is a complex expression, and it also means that changes to the value of finalValue within statement can affect execution of the loop.

Examples of for statements:

for I := 2 to 63 do

  if Data[I] > Max then

    Max := Data[I];

for I := ListBox1.Items.Count - 1 downto 0 do

  ListBox1.Items[I] := UpperCase(ListBox1.Items[I]);

for I := 1 to 10 do

  for J := 1 to 10 do

  begin

    X := 0;

    for K := 1 to 10 do

      X := X + Mat1[I, K] * Mat2[K, J];

    Mat[I, J] := X;

  end;

for C := Red to Blue do Check(C);


Blocks and scope: Overview
  Object Pascal Language Guide

Blocks and scope

Topic groups

Declarations and statements are organized into blocks, which define local namespaces (or scopes) for labels and identifiers. Blocks allow a single identifier, such as a variable name, to have different meanings in different parts of a program. Each block is part of the declaration of a program, function, or procedure; each program, function, or procedure declaration has one block.

Blocks

Scope


Blocks
  Object Pascal Language Guide

Blocks

Topic groups	See also

A block consists of a series of declarations followed by a compound statement. All declarations must occur together at the beginning of the block. So the form of a block is

declarations

begin

  statements

end

The declarations section can include, in any order, declarations for variables, constants (including resource strings), types, procedures, functions, and labels. In a program block, the declarations section can also include one or more exports clauses (see Dynamic-link libraries and packages).

For example, in a function declaration like

function UpperCase(const S: string): string;

var

  Ch: Char;

  L: Integer;

  Source, Dest: PChar;

begin

  ...

end;

the first line of the declaration is the function heading and all of the succeeding lines make up the block. Ch, L, Source, and Dest are local variables; their declarations apply only to the UpperCase function block and override—in this block only—any declarations of the same identifiers that may occur in the program block or in the interface or implementation section of a unit.


Scope
  Object Pascal Language Guide

Scope

Topic groups	See also

An identifier, such as a variable or function name, can be used only within the scope of its declaration. The location of a declaration determines its scope. An identifier declared within the declaration of a program, function, or procedure has a scope limited to the block in which it is declared. An identifier declared in the interface section of a unit has a scope that includes any other units or programs that use the unit where the declaration occurs. Identifiers with narrower scope—especially identifiers declared in functions and procedures—are sometimes called local, while identifiers with wider scope are called global.

The rules that determine identifier scope are summarized below.

If the identifier is declared in ...	its scope extends ...
the declaration of a program, function, or procedure	from the point where it is declared to the end of the current block, including all blocks enclosed within that scope.
the interface section of a unit	from the point where it is declared to the end of the unit, and to any other unit or program that uses that unit. (See Programs and units.)
the implementation section of a unit, but not within the block of any function or procedure	from the point where it is declared to the end of the unit. The identifier is available to any function or procedure in the unit, including the initialization and finalization sections, if present.
the definition of a record type (that is, the identifier is the name of a field in the record)	from the point of its declaration to the end of the record-type definition. (See Records.)
the definition of a class (that is, the identifier is the name of a data field property or method in the class)	from the point of its declaration to the end of the class-type definition, and also includes descendants of the class and the blocks of all methods in the class and its descendants. (See Classes and objects.)
Naming conflicts

When one block encloses another, the former is called the outer block and the latter the inner block. If an identifier declared in an outer block is redeclared in an inner block, the inner declaration overrides the outer one and determines the meaning of the identifier for the duration of the inner block. For example, if you declare a variable called MaxValue in the interface section of a unit, and then declare another variable with the same name in a function declaration within that unit, any unqualified occurrences of MaxValue in the function block are governed by the second, local declaration. Similarly, a function declared within another function creates a new, inner scope in which identifiers used by the outer function can be redeclared locally.

The use of multiple units further complicates the definition of scope. Each unit listed in a uses clause imposes a new scope that encloses the remaining units used and the program or unit containing the uses clause. The first unit in a uses clause represents the outermost scope and each succeeding unit represents a new scope inside the previous one. If two or more units declare the same identifier in their interface sections, an unqualified reference to the identifier selects the declaration in the innermost scope—that is, in the unit where the reference itself occurs, or, if that unit doesn’t declare the identifier, in the last unit in the uses clause that does declare the identifier.

The System unit is used automatically by every program or unit. The declarations in System, along with the predefined types, routines, and constants that the compiler understands automatically, always have the outermost scope.

You can override these rules of scope and by-pass an inner declaration by using a qualified identifier (see Qualified identifiers) or a with statement (see With statements).


Data types and variables: Overview
  Object Pascal Language Guide

Data types and variables

Topic groups	See also

A type is essentially a name for a kind of data. When you declare a variable you must specify its type, which determines the set of values the variable can hold and the operations that can be performed on it. Every expression returns data of a particular type, as does every function. Most functions and procedures require parameters of specific types.

Object Pascal is a “strongly typed” language, which means that it distinguishes a variety of data types and does not always allow you to substitute one type for another. This is usually beneficial because it lets the compiler treat data intelligently and validate your code more thoroughly, preventing hard-to-diagnose runtime errors. When you need greater flexibility, however, there are mechanisms to circumvent strong typing. These include typecasting, pointers, variants, variant parts in records, and absolute addressing of variables.


About types
  Object Pascal Language Guide

About types

Topic groups	See also

There are several ways to categorize Object Pascal data types:

Some types are predefined (or built-in); the compiler recognizes these automatically, without the need for a declaration. Almost all of the types documented in this language reference are predefined. Other types are created by declaration; these include user-defined types and the types defined in the product libraries.

Types can be classified as either fundamental or generic. The range and format of a fundamental type is the same in all implementations of Object Pascal, regardless of the underlying CPU and operating system. The range and format of a generic type is platform-specific and could vary across different implementations. Most predefined types are fundamental, but a handful of integer, character, string, and pointer types are generic. It’s a good idea to use generic types when possible, since they provide optimal performance and portability. However, changes in storage format from one implementation of a generic type to the next could cause compatibility problems—for example, if you are streaming data to a file.

Types can be classified as simple, string, structured, pointer, procedural, or variant. In addition, type identifiers themselves can be regarded as belonging to a special “type” because they can be passed as parameters to certain functions (such as High, Low, and SizeOf).

The outline below shows the taxonomy of Object Pascal data types.

simple

ordinal

integer

character

Boolean

enumerated

subrange

real

string

structured

set

array

record

file

class

class reference

interface

pointer

procedural

variant

(type identifier)

The standard function SizeOf operates on all variables and type identifiers. It returns an integer representing the amount of memory (in bytes) used to store data of the specified type. For example, SizeOf(Longint) returns 4, since a Longint variable uses four bytes of memory.

Type declarations are illustrated in the sections that follow. For general information about type declarations, see Declaring types.


Simple types: Overview
  Object Pascal Language Guide

Simple types

Topic groups	See also

Simple types, which include ordinal types and real types, define ordered sets of values.


Ordinal types: Overview
  Object Pascal Language Guide

Ordinal types

Topic groups	See also

Ordinal types include integer, character, Boolean, enumerated, and subrange types. An ordinal type defines an ordered set of values in which each value except the first has a unique predecessor and each value except the last has a unique successor. Further, each value has an ordinality which determines the ordering of the type. In most cases, if a value has ordinality n, its predecessor has ordinality n – 1 and its successor has ordinality n + 1.

For integer types, the ordinality of a value is the value itself.

Subrange types maintain the ordinalities of their base types.

For other ordinal types, by default the first value has ordinality 0, the next value has ordinality 1, and so forth. The declaration of an enumerated type can explicitly override this default.

Several predefined functions operate on ordinal values and type identifiers. The most important of them are summarized below.

Function	Parameter	Return value	Remarks
Ord	ordinal expression	ordinality of expression’s value	Does not take Int64 arguments.
Pred	ordinal expression	predecessor of expression’s value	Do not use on properties that have a write procedure.
Succ	ordinal expression	successor of expression’s value	Do not use on properties that have a write procedure.
High	ordinal type identifier or variable of ordinal type	highest value in type	Also operates on short-string types and arrays.
Low	ordinal type identifier or variable of ordinal type	lowest value in type	Also operates on short-string types and arrays.
For example, High(Byte) returns 255 because the highest value of type Byte is 255, and Succ(2) returns 3 because 3 is the successor of 2.

The standard procedures Inc and Dec increment and decrement the value of an ordinal variable. For example, Inc(I) is equivalent to I := Succ(I) and, if I is an integer variable, to I := I + 1.


Integer types
  Object Pascal Language Guide

Integer types

Topic groups	See also

An integer type represents a subset of the whole numbers. The generic integer types are Integer and Cardinal; use these whenever possible, since they result in the best performance for the underlying CPU and operating system. The table below gives their ranges and storage formats for the current 32-bit Object Pascal compiler.

Type	Range	Format
Integer	–2147483648..2147483647	signed 32-bit
Cardinal	0..4294967295	unsigned 32-bit
Fundamental integer types include Shortint, Smallint, Longint, Int64, Byte, Word, and Longword.

Type	Range	Format
Shortint	–128..127	signed 8-bit
Smallint	–32768..32767	signed 16-bit
Longint	–2147483648..2147483647	signed 32-bit
Int64	–2^63..2^63–1	signed 64-bit
Byte	0..255	unsigned 8-bit
Word	0..65535	unsigned 16-bit
Longword	0..4294967295	unsigned 32-bit
In general, arithmetic operations on integers return a value of type Integer—which, in its current implementation, is equivalent to the 32-bit Longint. Operations return a value of type Int64 only when performed on an Int64 operand. Hence the following code produces incorrect results.

var

  I: Integer;

  J: Int64;

  ...

I := High(Integer);

J := I + 1;

To get an Int64 return value in this situation, cast I as Int64:

 ...

J := Int64(I) + 1;

For more information, see Arithmetic operators.

Note:	Most standard routines that take integer arguments truncate Int64 values to 32 bits. However, the High, Low, Succ, Pred, Inc, Dec, IntToStr, and IntToHex routines fully support Int64 arguments. Also, the Round, Trunc, StrToInt64, and StrToInt64Def functions return Int64 values. A few routines—including Ord—cannot take Int64 values at all.

When you increment the last value or decrement the first value of an integer type, the result wraps around the beginning or end of the range. For example, the Shortint type has the range –128..127; hence, after execution of the code

var I: Shortint;

 ...

I := High(Shortint);

I := I + 1;

the value of I is –128. If compiler range-checking is enabled, however, this code generates a runtime error.


Character types
  Object Pascal Language Guide

Character types

Topic groups	See also

The fundamental character types are AnsiChar and WideChar. AnsiChar values are byte-sized (8-bit) characters ordered according to the locale character set which is possibly multibyte. AnsiChar was originally modeled after the ANSI character set (thus its name) but has now been broadened to refer to the current locale character set.

WideChar characters use more than one byte to represent every character. In the current implementations, WideChar is word-sized (16-bit) characters ordered according to the Unicode character set (note that it could be longer in future implementations). The first 256 Unicode characters correspond to the ANSI characters.

The generic character type is Char, which is equivalent to AnsiChar. Because the implementation of Char is subject to change, it’s a good idea to use the standard function SizeOf rather than a hard-coded constant when writing programs that may need to handle characters of different sizes.

A string constant of length 1, such as 'A', can denote a character value. The predefined function Chr returns the character value for any integer in the range of AnsiChar or WideChar; for example, Chr(65) returns the letter A.

Character values, like integers, wrap around when decremented or incremented past the beginning or end of their range (unless range-checking is enabled). For example, after execution of the code

var

  Letter: Char;

  I: Integer;

begin

  Letter := High(Letter);

  for I := 1 to 66 do

    Inc(Letter);

end;

Letter has the value A (ASCII 65).


Boolean types
  Object Pascal Language Guide

Boolean types

Topic groups	See also

The four predefined Boolean types are Boolean, ByteBool, WordBool, and LongBool. Boolean is the preferred type. The others exist to provide compatibility with other languages and operating system libraries.

A Boolean variable occupies one byte of memory, a ByteBool variable also occupies one byte, a WordBool variable occupies two bytes (one word), and a LongBool variable occupies four bytes (two words).

Boolean values are denoted by the predefined constants True and False. The following relationships hold.

Boolean	ByteBool, WordBool, LongBool
False < True	False <> True
Ord(False) = 0	Ord(False) = 0
Ord(True) = 1	Ord(True) <> 0
Succ(False) = True	Succ(False) = True
Pred(True) = False	Pred(False) = True
A value of type ByteBool, LongBool, or WordBool is considered True when its ordinality is nonzero. If such a value appears in a context where a Boolean is expected, the compiler automatically converts any value of nonzero ordinality to True.

The remarks above refer to the ordinality of Boolean values—not to the values themselves. In Object Pascal, Boolean expressions cannot be equated with integers or reals. Hence, if X is an integer variable, the statement

if X then ...;

generates a compilation error. Casting the variable to a Boolean type is unreliable, but each of the following alternatives will work.

if X <> 0 then ...;        { use longer expression that returns Boolean value }

var OK: Boolean            { use Boolean variable }

 ...

if X <> 0 then OK := True;

if OK then ...;


Enumerated types
  Object Pascal Language Guide

Enumerated types

Topic groups	See also

An enumerated type defines an ordered set of values by simply listing identifiers that denote these values. The values have no inherent meaning. To declare an enumerated type, use the syntax

type typeName = (val1, ..., valn)

where typeName and each val are valid identifiers. For example, the declaration

type Suit = (Club, Diamond, Heart, Spade);

defines an enumerated type called Suit whose possible values are Club, Diamond, Heart, and Spade, where Ord(Club) returns 0, Ord(Diamond) returns 1, and so forth.

When you declare an enumerated type, you are declaring each val to be a constant of type typeName. If the val identifiers are used for another purpose within the same scope, naming conflicts occur. For example, suppose you declare the type

type TSound = (Click, Clack, Clock);

Unfortunately, Click is also the name of a method defined for TControl and all of the objects in the VCL and/or CLX that descend from it. So if you’re writing an application and you create an event handler like

procedure TForm1.DBGrid1Enter(Sender: TObject);

var Thing: TSound;

begin

  ...

  Thing := Click;

  ...

end;

you’ll get a compilation error; the compiler interprets Click within the scope of the procedure as a reference to TForm’s Click method. You can work around this by qualifying the identifier; thus, if TSound is declared in MyUnit, you would use

Thing := MyUnit.Click;

A better solution, however, is to choose constant names that are not likely to conflict with other identifiers. Examples:

type

  TSound = (tsClick, tsClack, tsClock);

  TMyColor = (mcRed, mcBlue, mcGreen, mcYellow, mcOrange);

  Answer = (ansYes, ansNo, ansMaybe);

You can use the (val1, ..., valn) construction directly in variable declarations, as if it were a type name:

var MyCard: (Club, Diamond, Heart, Spade);

But if you declare MyCard this way, you can’t declare another variable within the same scope using these constant identifiers. Thus

var Card1: (Club, Diamond, Heart, Spade);

var Card2: (Club, Diamond, Heart, Spade);

generates a compilation error. But

var Card1, Card2: (Club, Diamond, Heart, Spade);

compiles cleanly, as does

type Suit = (Club, Diamond, Heart, Spade);

var

  Card1: Suit;

  Card2: Suit;

Enumerated types with explicitly assigned ordinality

By default, the ordinalities of enumerated values start from 0 and follow the sequence in which their identifiers are listed in the type declaration. You can override this by explicitly assigning ordinalities to some or all of the values in the declaration. To assign an ordinality to a value, follow its identifier with = constantExpression, where constantExpression is a constant expression that evaluates to an integer. For example,

type Size = (Small = 5, Medium = 10, Large = Small + Medium);

defines a type called Size whose possible values include Small, Medium, and Large, where Ord(Small) returns 5, Ord(Medium) returns 10, and Ord(Large) returns 15.

An enumerated type is, in effect, a subrange whose lowest and highest values correspond to the lowest and highest ordinalities of the constants in the declaration. In the example above, the Size type has 11 possible values whose ordinalities range from 5 to 15. (Hence the type array[Size] of Char represents an array of 11 characters.) Only three of these values have names, but the others are accessible through typecasts and through routines such as Pred, Succ, Inc, and Dec. In the following example, “anonymous” values in the range of Size are assigned to the variable X.

var X: Size;

X := Small;    // Ord(X) = 5

X := Size(6);  // Ord(X) = 6

Inc(X);        // Ord(X) = 7

Any value that isn’t explicitly assigned an ordinality has ordinality one greater than that of the previous value in the list. If the first value isn’t assigned an ordinality, its ordinality is 0. Hence, given the declaration

type SomeEnum = (e1, e2, e3 = 1);

SomeEnum has only two possible values: Ord(e1) returns 0, Ord(e2) returns 1, and Ord(e3) also returns 1; because e2 and e3 have the same ordinality, they represent the same value.


Subrange types
  Object Pascal Language Guide

Subrange types

Topic groups	See also

A subrange type represents a subset of the values in another ordinal type (called the base type). Any construction of the form Low..High, where Low and High are constant expressions of the same ordinal type and Low is less than High, identifies a subrange type that includes all values between Low and High. For example, if you declare the enumerated type

type TColors = (Red, Blue, Green, Yellow, Orange, Purple, White, Black);

you can then define a subrange type like

type TMyColors = Green..White;

Here TMyColors includes the values Green, Yellow, Orange, Purple, and White.

You can use numeric constants and characters (string constants of length 1) to define subrange types:

type

  SomeNumbers = -128..127;

  Caps = 'A'..'Z';

When you use numeric or character constants to define a subrange, the base type is the smallest integer or character type that contains the specified range.

The Low..High construction itself functions as a type name, so you can use it directly in variable declarations. For example,

var SomeNum: 1..500;

declares an integer variable whose value can be anywhere in the range from 1 to 500.

The ordinality of each value in a subrange is preserved from the base type. (In the first example above, if Color is a variable that holds the value Green, Ord(Color) returns 2 regardless of whether Color is of type TColors or TMyColors.) Values do not wrap around the beginning or end of a subrange, even if the base is an integer or character type; incrementing or decrementing past the boundary of a subrange simply converts the value to the base type. Hence, while

type Percentile = 0..99;

var I: Percentile;

...

I := 100;

produces an error,

...

I := 99;

Inc(I);

assigns the value 100 to I (unless compiler range-checking is enabled).

The use of constant expressions in subrange definitions introduces a syntactic difficulty. In any type declaration, when the first meaningful character after = is a left parenthesis, the compiler assumes that an enumerated type is being defined. Hence the code

const

  X = 50;

  Y = 10;

type

  Scale = (X - Y) * 2..(X + Y) * 2;

produces an error. Work around this problem by rewriting the type declaration to avoid the leading parenthesis:

type

  Scale = 2 * (X - Y)..(X + Y) * 2;


Real types
  Object Pascal Language Guide

Real types

Topic groups	See also

A real type defines a set of numbers that can be represented with floating-point notation. The table below gives the ranges and storage formats for the fundamental real types.

Type	Range	Significant digits	Size in bytes
Real48	2.9 x 10^–39 .. 1.7 x 10^38	11–12	6
Single	1.5 x 10^–45 .. 3.4 x 10^38	7–8	4
Double	5.0 x 10^–324 .. 1.7 x 10^308	15–16	8
Extended	3.6 x 10^–4951 .. 1.1 x 10^4932	19–20	10
Comp	–2^63+1 .. 2^63 –1	19–20	8
Currency	–922337203685477.5808.. 922337203685477.5807	19–20	8
The generic type Real, in its current implementation, is equivalent to Double.

Type	Range	Significant digits	Size in bytes
Real	5.0 x 10^–324 .. 1.7 x 10^308	15–16	8
Note:	The six-byte Real48 type was called Real in earlier versions of Object Pascal. If you are recompiling code that uses the older, six-byte Real type, you may want to change it to Real48. You can also use the {$REALCOMPATIBILITY ON} compiler directive to turn Real back into the six-byte type.

The following remarks apply to fundamental real types.

Real48 is maintained for backward compatibility. Since its storage format is not native to the Intel CPU family, it results in slower performance than other floating-point types.

Extended offers greater precision than other real types but is less portable. Be careful using Extended if you are creating data files to share across platforms.

The Comp (computational) type is native to the Intel CPU and represents a 64-bit integer. It is classified as a real, however, because it does not behave like an ordinal type. (For example, you cannot increment or decrement a Comp value.) Comp is maintained for backward compatibility only. Use the Int64 type for better performance.

Currency is a fixed-point data type that minimizes rounding errors in monetary calculations. It is stored as a scaled 64-bit integer with the four least-significant digits implicitly representing decimal places. When mixed with other real types in assignments and expressions, Currency values are automatically divided or multiplied by 10000.


About string types
  Object Pascal Language Guide

String types

Topic groups	See also

A string represents a sequence of characters. Object Pascal supports the following predefined string types.

Type	Maximum length	Memory required	Used for
ShortString	255 characters	2 to 256 bytes	backward compatibility
AnsiString	~2^31 characters	4 bytes to 2GB	8-bit (ANSI) characters
WideString	~2^30 characters	4 bytes to 2GB	Unicode characters;  multiuser servers and multi-language applications
AnsiString, sometimes called the long string, is the preferred type for most purposes.

String types can be mixed in assignments and expressions; the compiler automatically performs required conversions. But strings passed by reference to a function or procedure (as var and out parameters) must be of the appropriate type. Strings can be explicitly cast to a different string type (see Typecasts).

The reserved word string functions like a generic type identifier. For example,

var S: string;

creates a variable S that holds a string. In the default {$H+} state, the compiler interprets string (when it appears without a bracketed number after it) as AnsiString. Use the {$H–} directive to turn string into ShortString.

The standard function Length returns the number of characters in a string. The SetLength procedure adjusts the length of a string.

Comparison of strings is defined by the ordering of the characters in corresponding positions. Between strings of unequal length, each character in the longer string without a corresponding character in the shorter string takes on a greater-than value. For example, “AB” is greater than “A”; that is, 'AB' > 'A' returns True. Zero-length strings hold the lowest values.

You can index a string variable just as you would an array. If S is a string variable and i an integer expression, S[i] represents the ith character—or, strictly speaking, the ith byte—in S. For a ShortString or AnsiString, S[i] is of type AnsiChar; for a WideString, S[i] is of type WideChar. The statement MyString[2] := 'A'; assigns the value A to the second character of MyString. The following code uses the standard UpCase function to convert MyString to uppercase.

var I: Integer;

begin

  I := Length(MyString);

  while I > 0 do

  begin

    MyString[I] := UpCase(MyString[I]);

    I := I - 1;

  end;

end;

Be careful indexing strings in this way, since overwriting the end of a string can cause access violations. Also, avoid passing long-string indexes as var parameters, because this results in inefficient code.

You can assign the value of a string constant—or any other expression that returns a string—to a variable. The length of the string changes dynamically when the assignment is made. Examples:

MyString := 'Hello world!';

MyString := 'Hello ' + 'world';

MyString := MyString + '!';

MyString := ' ';               { space }

MyString := '';                { empty string }


Short strings
  Object Pascal Language Guide

Short strings

Topic groups	See also

A ShortString is 0 to 255 characters long. While the length of a ShortString can change dynamically, its memory is a statically allocated 256 bytes; the first byte stores the length of the string, and the remaining 255 bytes are available for characters. If S is a ShortString variable, Ord(S[0]), like Length(S), returns the length of S; assigning a value to S[0], like calling SetLength, changes the length of S. ShortString uses 8-bit ANSI characters and is maintained for backward compatibility only.

Object Pascal supports short-string types—in effect, subtypes of ShortString—whose maximum length is anywhere from 0 to 255 characters. These are denoted by a bracketed numeral appended to the reserved word string. For example,

var MyString: string[100];

creates a variable called MyString whose maximum length is 100 characters. This is equivalent to the declarations

type CString = string[100];

var MyString: CString;

Variables declared in this way allocate only as much memory as the type requires—that is, the specified maximum length plus one byte. In our example, MyString uses 101 bytes, as compared to 256 bytes for a variable of the predefined ShortString type.

When you assign a value to a short-string variable, the string is truncated if it exceeds the maximum length for the type.

The standard functions High and Low operate on short-string type identifiers and variables. High returns the maximum length of the short-string type, while Low returns zero.


Long strings
  Object Pascal Language Guide

Long strings

Topic groups	See also

AnsiString, also called a long string, represents a dynamically allocated string whose maximum length is limited only by available memory. It uses 8-bit ANSI characters.

A long-string variable is a pointer occupying four bytes of memory. When the variable is empty—that is, when it contains a zero-length string—the pointer is nil and the string uses no additional storage. When the variable is nonempty, it points to a dynamically allocated block of memory that contains the string value, a 32-bit length indicator, and a 32-bit reference count. This memory is allocated on the heap, but its management is entirely automatic and requires no user code.

Because long-string variables are pointers, two or more of them can reference the same value without consuming additional memory. The compiler exploits this to conserve resources and execute assignments faster. Whenever a long-string variable is destroyed or assigned a new value, the reference count of the old string (the variable’s previous value) is decremented and the reference count of the new value (if there is one) is incremented; if the reference count of a string reaches zero, its memory is deallocated. This process is called reference-counting. When indexing is used to change the value of a single character in a string, a copy of the string is made if—but only if—its reference count is greater than one. This is called copy-on-write semantics.


WideString
  Object Pascal Language Guide

WideString

Topic groups	See also

The WideString type represents a dynamically allocated string of 16-bit Unicode characters. In most respects it is similar to AnsiString.

On Win32, WideString is compatible with the COM BSTR type. Borland development tools have support features that convert AnsiString values to WideString, but you may need to explicitly cast or convert your strings to WideString.

About extended character sets


About extended character sets
  Object Pascal Language Guide

About extended character sets

Topic groups	See also

Windows and Linux both support single-byte and multibyte character sets as well as Unicode. With a single-byte character set (SBCS), each byte in a string represents one character. The ANSI character set used by many Western operating systems is a single-byte character set.

In a multibyte character set (MBCS), some characters are represented by one byte and others by more than one byte. The first byte of a multibyte character is called the lead byte. In general, the lower 128 characters of a multibyte character set map to the 7-bit ASCII characters, and any byte whose ordinal value is greater than 127 is the lead byte of a multibyte character. Only single-byte characters can contain the null value (#0). Multibyte character sets—especially double-byte character sets (DBCS)—are widely used for Asian languages, while the UTF-8 character set used by Linux is a multibyte encoding of Unicode.

In the Unicode character set, each character is represented by two bytes. Thus a Unicode string is a sequence not of individual bytes but of two-byte words. Unicode characters and strings are also called wide characters and wide character strings. The first 256 Unicode characters map to the ANSI character set. The Windows operating system supports Unicode (UCS-2). The Linux operating system supports UCS-4, a superset of UCS-2. Delphi/Kylix supports UCS-2 on both platforms.

Object Pascal supports single-byte and multibyte characters and strings through the Char, PChar, AnsiChar, PAnsiChar, and AnsiString types. Indexing of multibyte strings is not reliable, since S[i] represents the ith byte (not necessarily the ith character) in S. However, the standard string-handling functions have multibyte-enabled counterparts that also implement locale-specific ordering for characters. (Names of multibyte functions usually start with Ansi-. For example, the multibyte version of StrPos is AnsiStrPos.) Multibyte character support is operating-system dependent and based on the current locale.

Object Pascal supports Unicode characters and strings through the WideChar, PWideChar, and WideString types.


Working with null-terminated strings
  Object Pascal Language Guide

Working with null-terminated strings

Topic groups	See also

Many programming languages, including C and C++, lack a dedicated string data type. These languages, and environments that are built with them, rely on null-terminated strings. A null-terminated string is a zero-based array of characters that ends with NULL (#0); since the array has no length indicator, the first NULL character marks the end of the string. You can use Object Pascal constructions and special routines in the SysUtils unit (see Standard routines and I/O) to handle null-terminated strings when you need to share data with systems that use them.

For example, the following type declarations could be used to store null-terminated strings.

type

  TIdentifier = array[0..15] of Char;

  TFileName = array[0..259] of Char;

  TMemoText = array[0..1023] of WideChar;

With extended syntax enabled ({$X+}), you can assign a string constant to a statically allocated zero-based character array. (Dynamic arrays won’t work for this purpose.) If you initialize an array constant with a string that is shorter than the declared length of the array, the remaining characters are set to #0.

Using pointers, arrays, and string constants

Mixing Pascal strings and null-terminated strings


Using pointers, arrays, and string constants
  Object Pascal Language Guide

Using pointers, arrays, and string constants

Topic groups	See also

To manipulate null-terminated strings, it is often necessary to use pointers. (See Pointers and pointer types.) String constants are assignment-compatible with the PChar and PWideChar types, which represent pointers to null-terminated arrays of Char and WideChar values. For example,

var P: PChar;

 ...

P := 'Hello world!';

points P to an area of memory that contains a null-terminated copy of “Hello world!” This is equivalent to

const TempString: array[0..12] of Char = 'Hello world!'#0;

var P: PChar;

 ...

P := @TempString;

You can also pass string constants to any function that takes value or const parameters of type PChar or PWideChar—for example StrUpper('Hello world!'). As with assignments to a PChar, the compiler generates a null-terminated copy of the string and gives the function a pointer to that copy. Finally, you can initialize PChar or PWideChar constants with string literals, alone or in a structured type. Examples:

const

Message: PChar = 'Program terminated';

  Prompt: PChar = 'Enter values: ';

  Digits: array[0..9] of PChar = (

    'Zero', 'One', 'Two', 'Three', 'Four',

    'Five', 'Six', 'Seven', 'Eight', 'Nine');

Zero-based character arrays are compatible with PChar and PWideChar. When you use a character array in place of a pointer value, the compiler converts the array to a pointer constant whose value corresponds to the address of the first element of the array. For example,

var

  MyArray: array[0..32] of Char;

  MyPointer: PChar;

begin

  MyArray := 'Hello';

  MyPointer := MyArray;

  SomeProcedure(MyArray);

  SomeProcedure(MyPointer);

end;

This code calls SomeProcedure twice with the same value.

A character pointer can be indexed as if it were an array. In the example above, MyPointer[0] returns H. The index specifies an offset added to the pointer before it is dereferenced. (For PWideChar variables, the index is automatically multiplied by two.) Thus, if P is a character pointer, P[0] is equivalent to P^ and specifies the first character in the array, P[1] specifies the second character in the array, and so forth; P[-1] specifies the “character” immediately to the left of P[0]. The compiler performs no range checking on these indexes.

The StrUpper function illustrates the use of pointer indexing to iterate through a null-terminated string:

function StrUpper(Dest, Source: PChar; MaxLen: Integer): PChar;

var

  I: Integer;

begin

  I := 0;

  while (I < MaxLen) and (Source[I] <> #0) do

  begin

    Dest[I] := UpCase(Source[I]);

    Inc(I);

  end;

  Dest[I] := #0;

  Result := Dest;

end;


Mixing Pascal strings and null-terminated strings
  Object Pascal Language Guide

Mixing Pascal strings and null-terminated strings

Topic groups	See also

You can mix long strings (AnsiString values) and null-terminated strings (PChar values) in expressions and assignments, and you can pass PChar values to functions or procedures that take long-string parameters. The assignment S := P, where S is a string variable and P is a PChar expression, copies a null-terminated string into a long string.

In a binary operation, if one operand is a long string and the other a PChar, the PChar operand is converted to a long string.

You can cast a PChar value as a long string. This is useful when you want to perform a string operation on two PChar values. For example,

S := string(P1) + string(P2);

You can also cast a long string as a null-terminated string. The following rules apply.

If S is a long-string expression, PChar(S) casts S as a null-terminated string; it returns a pointer to the first character in S.

On Windows: For example, if Str1 and Str2 are long strings, you could call the Win32 API MessageBox function like this:

MessageBox(0, PChar(Str1), PChar(Str2), MB_OK);

(The declaration of MessageBox is in the Windows interface unit.)

On Linux: For example, if Str is a long string, you could call the opendir system function like this:

opendir(PChar(Str));

(The declaration of opendir is in the Libc interface unit.)

You can also use Pointer(S) to cast a long string to an untyped pointer. But if S is empty, the typecast returns nil.

When you cast a long-string variable to a pointer, the pointer remains valid until the variable is assigned a new value or goes out of scope. If you cast any other long-string expression to a pointer, the pointer is valid only within the statement where the typecast is performed.

When you cast a long-string expression to a pointer, the pointer should usually be considered read-only. You can safely use the pointer to modify the long string only when all of the following conditions are satisfied.

The expression cast is a long-string variable.

The string is not empty.

The string is unique—that is, has a reference count of one. To guarantee that the string is unique, call the SetLength, SetString, or UniqueString procedure.

The string has not been modified since the typecast was made.

The characters modified are all within the string. Be careful not to use an out-of-range index on the pointer.

The same rules apply when mixing WideString values with PWideChar values.


Structured types: Overview
  Object Pascal Language Guide

Structured types

Topic groups	See also

Instances of a structured type hold more than one value. Structured types include sets, arrays, records, and files as well as class, class-reference, and interface types. Except for sets, which hold ordinal values only, structured types can contain other structured types; a type can have unlimited levels of structuring.

By default, the values in a structured type are aligned on word or double-word boundaries for faster access. When you declare a structured type, you can include the reserved word packed to implement compressed data storage. For example,

type TNumbers = packed array[1..100] of Real;

Using packed slows data access and, in the case of a character array, affects type compatibility. For more information, see Memory management.


Sets
  Object Pascal Language Guide

Sets

Topic groups	See also

A set is a collection of values of the same ordinal type. The values have no inherent order, nor is it meaningful for a value to be included twice in a set.

The range of a set type is the power set of a specific ordinal type, called the base type; that is, the possible values of the set type are all the subsets of the base type, including the empty set. The base type can have no more than 256 possible values, and their ordinalities must fall between 0 and 255. Any construction of the form

set of baseType

where baseType is an appropriate ordinal type, identifies a set type.

Because of the size limitations for base types, set types are usually defined with subranges. For example, the declarations

type

  TSomeInts = 1..250;

  TIntSet = set of TSomeInts;

create a set type called TIntSet whose values are collections of integers in the range from 1 to 250. You could accomplish the same thing with

type TIntSet = set of 1..250;

Given this declaration, you can create a sets like this:

var Set1, Set2: TIntSet;

 ...

Set1 := [1, 3, 5, 7, 9];

Set2 := [2, 4, 6, 8, 10]

You can also use the set of ... construction directly in variable declarations:

var MySet: set of 'a'..'z';

 ...

MySet := ['a','b','c'];

Other examples of set types include

set of Byte

set of (Club, Diamond, Heart, Spade)

set of Char;

The in operator tests set membership:

if 'a' in MySet then ... { do something } ;

Every set type can hold the empty set, denoted by [].


Arrays: Overview
  Object Pascal Language Guide

Arrays

Topic groups	See also

An array represents an indexed collection of elements of the same type (called the base type). Because each element has a unique index, arrays, unlike sets, can meaningfully contain the same value more than once. Arrays can be allocated statically or dynamically.

Static arrays

Dynamic arrays


Static arrays
  Object Pascal Language Guide

Static arrays

Topic groups	See also

Static array types are denoted by constructions of the form

array[indexType1, ..., indexTypen] of baseType

where each indexType is an ordinal type whose range does not exceed 2GB. Since the indexTypes index the array, the number of elements an array can hold is limited by the product of the sizes of the indexTypes. In practice, indexTypes are usually integer subranges.

In the simplest case of a one-dimensional array, there is only a single indexType. For example,

var MyArray: array[1..100] of Char;

declares a variable called MyArray that holds an array of 100 character values. Given this declaration, MyArray[3] denotes the third character in MyArray. If you create a static array but don’t assign values to all its elements, the unused elements are still allocated and contain random data; they are like uninitialized variables.

A multidimensional array is an array of arrays. For example,

type TMatrix = array[1..10] of array[1..50] of Real;

is equivalent to

type TMatrix = array[1..10, 1..50] of Real;

Whichever way TMatrix is declared, it represents an array of 500 real values. A variable MyMatrix of type TMatrix can be indexed like this: MyMatrix[2,45]; or like this: MyMatrix[2][45]. Similarly,

packed array[Boolean,1..10,TShoeSize] of Integer;

is equivalent to

packed array[Boolean] of packed array[1..10] of packed array[TShoeSize] of Integer;

The standard functions Low and High operate on array type identifiers and variables. They return the low and high bounds of the array’s first index type. The standard function Length returns the number of elements in the array’s first dimension.

A one-dimensional, packed, static array of Char values is called a packed string. Packed-string types are compatible with string types and with other packed-string types that have the same number of elements. See Type compatibility and identity.

An array type of the form array[0..x] of Char is called a zero-based character array. Zero-based character arrays are used to store null-terminated strings and are compatible with PChar values. See Working with null-terminated strings.


Dynamic arrays
  Object Pascal Language Guide

Dynamic arrays

Topic groups	See also

Dynamic arrays do not have a fixed size or length. Instead, memory for a dynamic array is reallocated when you assign a value to the array or pass it to the SetLength procedure. Dynamic-array types are denoted by constructions of the form

array of baseType

For example,

var MyFlexibleArray: array of Real;

declares a one-dimensional dynamic array of reals. The declaration does not allocate memory for MyFlexibleArray. To create the array in memory, call SetLength. For example, given the declaration above,

SetLength(MyFlexibleArray, 20);

allocates an array of 20 reals, indexed 0 to 19. Dynamic arrays are always integer-indexed, always starting from 0.

Dynamic-array variables are implicitly pointers and are managed by the same reference-counting technique used for long strings. To deallocate a dynamic array, assign nil to a variable that references the array or pass the variable to Finalize; either of these methods disposes of the array, provided there are no other references to it. Dynamic arrays of length 0 have the value nil. Do not apply the dereference operator (^) to a dynamic-array variable or pass it to the New or Dispose procedure.

If X and Y are variables of the same dynamic-array type, X := Y points X to the same array as Y. (There is no need to allocate memory for X before performing this operation.) Unlike strings and static arrays, dynamic arrays are not automatically copied before they are written to. For example, after this code executes—

var

  A, B: array of Integer;

begin

  SetLength(A, 1);

  A[0] := 1;

  B := A;

  B[0] := 2;

end;

—the value of A[0] is 2. (If A and B were static arrays, A[0] would still be 1.)

Assigning to a dynamic-array index (for example, MyFlexibleArray[2] := 7) does not reallocate the array. Out-of-range indexes are not reported at compile time.

When dynamic-array variables are compared, their references are compared, not their array values. Thus, after execution of the code

var

  A, B: array of Integer;

begin

  SetLength(A, 1);

  SetLength(B, 1);

  A[0] := 2;

  B[0] := 2;

end;

A = B returns False but A[0] = B[0] returns True.

To truncate a dynamic array, pass it to SetLength or Copy and assign the result back to the array variable. (The SetLength procedure is usually faster.) For example, if A is a dynamic array, A := SetLength(A, 0, 20) truncates all but the first 20 elements of A.

Once a dynamic array has been allocated, you can pass it to the standard functions Length, High, and Low. Length returns the number of elements in the array, High returns the array’s highest index (that is, Length – 1), and Low returns 0. In the case of a zero-length array, High returns –1 (with the anomalous consequence that High < Low).

Note:	In some function and procedure declarations, array parameters are represented as array of baseType, without any index types specified. For example,

function CheckStrings(A: array of string): Boolean;

This indicates that the function operates on all arrays of the specified base type, regardless of their size, how they are indexed, or whether they are allocated statically or dynamically. See Open array parameters.

Multidimensional dynamic arrays


Multidimensional dynamic arrays
  Object Pascal Language Guide

Multidimensional dynamic arrays

Topic groups	See also

To declare multidimensional dynamic arrays, use iterated array of ... constructions. For example,

type TMessageGrid = array of array of string;

var Msgs: TMessageGrid;

declares a two-dimensional array of strings. To instantiate this array, call SetLength with two integer arguments. For example, if I and J are integer-valued variables,

SetLength(Msgs,I,J);

allocates an I-by-J array, and Msgs[0,0] denotes an element of that array.

You can create multidimensional dynamic arrays that are not rectangular. The first step is to call SetLength, passing it parameters for the first n dimensions of the array. For example,

var Ints: array of array of Integer;

SetLength(Ints,10);

allocates ten rows for Ints but no columns. Later, you can allocate the columns one at a time (giving them different lengths); for example

SetLength(Ints[2], 5);

makes the third column of Ints five integers long. At this point (even if the other columns haven’t been allocated) you can assign values to the third column—for example, Ints[2,4] := 6.

The following example uses dynamic arrays (and the IntToStr function declared in the SysUtils unit) to create a triangular matrix of strings.

var

  A : array of array of string;

  I, J : Integer;

begin

  SetLength(A, 10);

  for I := Low(A) to High(A) do

  begin

    SetLength(A[I], I);

    for J := Low(A[I]) to High(A[I]) do

      A[I,J] := IntToStr(I) + ',' + IntToStr(J) + ' ';

  end;

end;


Array types and assignments
  Object Pascal Language Guide

Array types and assignments

Topic groups	See also

Arrays are assignment-compatible only if they are of the same type. Because Pascal uses name-equivalence for types, the following code will not compile.

var

  Int1: array[1..10] of Integer;

  Int2: array[1..10] of Integer;

  ...

Int1 := Int2;

To make the assignment work, declare the variables as

var Int1, Int2: array[1..10] of Integer;

or

type IntArray = array[1..10] of Integer;

var

  Int1: IntArray;

  Int2: IntArray;


About records
  Object Pascal Language Guide

Records

Topic groups	See also

A record (analogous to a structure in some languages) represents a heterogeneous set of elements. Each element is called a field; the declaration of a record type specifies a name and type for each field. The syntax of a record type declaration is

type recordTypeName = record

  fieldList1: type1;

  ...

  fieldListn: typen;

end

where recordTypeName is a valid identifier, each type denotes a type, and each fieldList is a valid identifier or a comma-delimited list of identifiers. The final semicolon is optional.

For example, the following declaration creates a record type called TDateRec.

type

  TDateRec = record

    Year: Integer;

    Month: (Jan, Feb, Mar, Apr, May, Jun,

            Jul, Aug, Sep, Oct, Nov, Dec);

    Day: 1..31;

  end;

Each TDateRec contains three fields: an integer value called Year, a value of an enumerated type called Month, and another integer between 1 and 31 called Day. The identifiers Year, Month, and Day are the field designators for TDateRec, and they behave like variables. The TDateRec type declaration, however, does not allocate any memory for the Year, Month, and Day fields; memory is allocated when you instantiate the record, like this:

var Record1, Record2: TDateRec;

This variable declaration creates two instances of TDateRec, called Record1 and Record2.

You can access the fields of a record by qualifying the field designators with the record’s name:

Record1.Year := 1904;

Record1.Month := Jun;

Record1.Day := 16;

Or use a with statement:

with Record1 do

begin

  Year := 1904;

  Month := Jun;

  Day := 16;

end;

You can now copy the values of Record1’s fields to Record2:

Record2 := Record1;

Because the scope of a field designator is limited to the record in which it occurs, you don’t have to worry about naming conflicts between field designators and other variables.

Instead of defining record types, you can use the record ... construction directly in variable declarations:

var S: record

  Name: string;

  Age: Integer;

end;

However, a declaration like this largely defeats the purpose of records, which is to avoid repetitive coding of similar groups of variables. Moreover, separately declared records of this kind will not be assignment-compatible, even if their structures are identical.

Variant parts in records


Variant parts in records
  Object Pascal Language Guide

Variant parts in records

Topic groups	See also

A record type can have a variant part, which looks like a case statement. The variant part must follow the other fields in the record declaration.

To declare a record type with a variant part, use the following syntax.

type recordTypeName = record

  fieldList1: type1;

  ...

  fieldListn: typen;

case tag: ordinalType of

  constantList1: (variant1);

  ...

  constantListn: (variantn);

end;

The first part of the declaration—up to the reserved word case—is the same as that of a standard record type. The remainder of the declaration—from case to the optional final semicolon—is called the variant part. In the variant part,

tag is optional and can be any valid identifier. If you omit tag, omit the colon (:) after it as well.

ordinalType denotes an ordinal type.

Each constantList is a constant denoting a value of type ordinalType, or a comma-delimited list of such constants. No value can be represented more than once in the combined constantLists.

Each variant is a comma-delimited list of declarations resembling the fieldList: type constructions in the main part of the record type. That is, a variant has the form

fieldList1: type1;

 ...

fieldListn: typen;

where each fieldList is a valid identifier or comma-delimited list of identifiers, each type denotes a type, and the final semicolon is optional. The types must not be long strings, dynamic arrays, variants (that is, Variant types), or interfaces, nor can they be structured types that contain long strings, dynamic arrays, variants, or interfaces; but they can be pointers to these types.

Records with variant parts are complicated syntactically but deceptively simple semantically. The variant part of a record contains several variants which share the same space in memory. You can read or write to any field of any variant at any time; but if you write to a field in one variant and then to a field in another variant, you may be overwriting your own data. The tag, if there is one, functions as an extra field (of type ordinalType) in the non-variant part of the record.

Variant parts have two purposes. First, suppose you want to create a record type that has fields for different kinds of data, but you know that you will never need to use all of the fields in a single record instance. For example,

type

  TEmployee = record

  FirstName, LastName: string[40];

  BirthDate: TDate;

  case Salaried: Boolean of

    True: (AnnualSalary: Currency);

    False: (HourlyWage: Currency);

end;

The idea here is that every employee has either a salary or an hourly wage, but not both. So when you create an instance of TEmployee, there is no reason to allocate enough memory for both fields. In this case, the only difference between the variants is in the field names, but the fields could just as easily have been of different types. Consider some more complicated examples:

type

  TPerson = record

  FirstName, LastName: string[40];

  BirthDate: TDate;

  case Citizen: Boolean of

    True: (Birthplace: string[40]);

    False: (Country: string[20];

            EntryPort: string[20];

            EntryDate, ExitDate: TDate);

  end;

type

  TShapeList = (Rectangle, Triangle, Circle, Ellipse, Other);

  TFigure = record

    case TShapeList of

      Rectangle: (Height, Width: Real);

      Triangle: (Side1, Side2, Angle: Real);

      Circle: (Radius: Real);

      Ellipse, Other: ();

  end;

For each record instance, the compiler allocates enough memory to hold all the fields in the largest variant. The optional tag and the constantLists (like Rectangle, Triangle, and so forth in the last example above) play no role in the way the compiler manages the fields; they are there only for the convenience of the programmer.

The second reason for variant parts is that they let you treat the same data as belonging to different types, even in cases where the compiler would not allow a typecast. For example, if you have a 64-bit Real as the first field in one variant and a 32-bit Integer as the first field in another, you can assign a value to the Real field and then read back the first 32 bits of it as the value of the Integer field (passing it, say, to a function that requires integer parameters).


File types
  Object Pascal Language Guide

File types

Topic groups	See also

A file is an ordered set of elements of the same type. Standard I/O routines use the predefined TextFile or Text type, which represents a file containing characters organized into lines. For more information about file input and output, see Standard routines and I/O.

To declare a file type, use the syntax

type fileTypeName = file of type

where fileTypeName is any valid identifier and type is a fixed-size type. Pointer types—whether implicit or explicit—are not allowed, so a file cannot contain dynamic arrays, long strings, classes, objects, pointers, variants, other files, or structured types that contain any of these.

For example,

type

  PhoneEntry = record

    FirstName, LastName: string[20];

    PhoneNumber: string[15];

    Listed: Boolean;

  end;

  PhoneList = file of PhoneEntry;

declares a file type for recording names and telephone numbers.

You can also use the file of ... construction directly in a variable declaration. For example,

var List1: file of PhoneEntry;

The word file by itself indicates an untyped file:

var DataFile: file;

For more information, see Untyped files.

Files are not allowed in arrays or records.


Pointers and pointer types
  Object Pascal Language Guide

Pointers and pointer types

Topic groups	See also

A pointer is a variable that denotes a memory address. When a pointer holds the address of another variable, we say that it points to the location of that variable in memory or to the data stored there. In the case of an array or other structured type, a pointer holds the address of the first element in the structure.

Pointers are typed to indicate the kind of data stored at the addresses they hold. The general-purpose Pointer type can represent a pointer to any data, while more specialized pointer types reference only specific types of data. Pointers occupy four bytes of memory.

Overview of pointers

Pointer types


Overview of pointers
  Object Pascal Language Guide

Overview of pointers

Topic groups	See also

To see how pointers work, look at the following example.

1    var

2      X, Y: Integer;   // X and Y are Integer variables

3      P: ^Integer;     // P points to an Integer

4    begin

5      X := 17;         // assign a value to X

6      P := @X;         // assign the address of X to P

7      Y := P^;         // dereference P; assign the result to Y

8    end;

Line 2 declares X and Y as variables of type Integer. Line 3 declares P as a pointer to an Integer value; this means that P can point to the location of X or Y. Line 5 assigns a value to X, and line 6 assigns the address of X (denoted by @X) to P. Finally, line 7 retrieves the value at the location pointed to by P (denoted by ^P) and assigns it to Y. After this code executes, X and Y have the same value, namely 17.

The @ operator, which we have used here to take the address of a variable, also operates on functions and procedures. For more information, see The @ operator and Procedural types in statements and expressions.

The symbol ^ has two purposes, both of which are illustrated in our example. When it appears before a type identifier—

^typeName

—it denotes a type that represents pointers to variables of type typeName. When it appears after a pointer variable—

pointer^

—it dereferences the pointer; that is, it returns the value stored at the memory address held by the pointer.

Our example may seem like a roundabout way of copying the value of one variable to another—something that we could have accomplished with a simple assignment statement. But pointers are useful for several reasons. First, understanding pointers will help you to understand Object Pascal, since pointers often operate behind the scenes in code where they don’t appear explicitly. Any data type that requires large, dynamically allocated blocks of memory uses pointers. Long-string variables, for instance, are implicitly pointers, as are class variables. Moreover, some advanced programming techniques require the use of pointers.

Finally, pointers are sometimes the only way to circumvent Object Pascal’s strict data typing. By referencing a variable with an all-purpose Pointer, casting the Pointer to a more specific type, and then dereferencing it, you can treat the data stored by any variable as if it belonged to any type. For example, the following code assigns data stored in a real variable to an integer variable.

type

  PInteger = ^Integer;

var

  R: Single;

  I: Integer;

  P: Pointer;

  PI: PInteger;

begin

  ...

  P := @R;

  PI := PInteger(P);

  I := PI^;

end;

Of course, reals and integers are stored in different formats. This assignment simply copies raw binary data from R to I, without converting it.

In addition to assigning the result of an @ operation, you can use several standard routines to give a value to a pointer. The New and GetMem procedures assign a memory address to an existing pointer, while the Addr and Ptr functions return a pointer to a specified address or variable.

Dereferenced pointers can be qualified and can function as qualifiers, as in the expression P1^.Data^.

The reserved word nil is a special constant that can be assigned to any pointer. When nil is assigned to a pointer, the pointer doesn’t reference anything.


About pointer types
  Object Pascal Language Guide

Pointer types

Topic groups	See also

You can declare a pointer to any type, using the syntax

type pointerTypeName = ^type

When you define a record or other data type, it’s a common practice also to define a pointer to that type. This makes it easy to manipulate instances of the type without copying large blocks of memory.

Standard pointer types exist for many purposes. The most versatile is Pointer, which can point to data of any kind. But a Pointer variable cannot be dereferenced; placing the ^ symbol after a Pointer variable causes a compilation error. To access the data referenced by a Pointer variable, first cast it to another pointer type and then dereference it.

Character pointers

Other standard pointer types


Character pointers
  Object Pascal Language Guide

Character pointers

Topic groups	See also

The fundamental types PAnsiChar and PWideChar represent pointers to AnsiChar and WideChar values, respectively. The generic PChar represents a pointer to a Char (that is, in its current implementation, to an AnsiChar). These character pointers are used to manipulate null-terminated strings. (See Working with null-terminated strings.)


Other standard pointer types
  Object Pascal Language Guide

Other standard pointer types

Topic groups	See also

The System and SysUtils units declare many standard pointer types that are commonly used.

Pointer type	Points to variables of type
PAnsiString, PString	AnsiString
PByteArray	TByteArray (declared in SysUtils). Used to typecast dynamically allocated memory for array access.
PCurrency, PDouble, PExtended, PSingle	Currency, Double, Extended, Single
PInteger	Integer
POleVariant	OleVariant
PShortString	ShortString. Useful when porting legacy code that uses the old PString type.
PTextBuf	TTextBuf (declared in SysUtils). TTextBuf is the internal buffer type in a TTextRec file record.)
PVarRec	TVarRec (declared in System)
PVariant	Variant
PWideString	WideString
PWordArray	TWordArray (declared in SysUtils). Used to typecast dynamically allocated memory for arrays of 2-byte values.


Procedural types: Overview
  Object Pascal Language Guide

Procedural types

Topic groups	See also

Procedural types allow you to treat procedures and functions as values that can be assigned to variables or passed to other procedures and functions. For example, suppose you define a function called Calc that takes two integer parameters and returns an integer:

function Calc(X,Y: Integer): Integer;

You can assign the Calc function to the variable F:

var F: function(X,Y: Integer): Integer;

F := Calc;

If you take any procedure or function heading and remove the identifier after the word procedure or function, what’s left is the name of a procedural type. You can use such type names directly in variable declarations (as in the example above) or to declare new types:

type

  TIntegerFunction = function: Integer;

  TProcedure = procedure;

  TStrProc = procedure(const S: string);

  TMathFunc = function(X: Double): Double;

var

  F: TIntegerFunction;             { F is a parameterless function that returns an integer }

  Proc: TProcedure;                { Proc is a parameterless procedure }

  SP: TStrProc;                    { SP is a procedure that takes a string parameter }

  M: TMathFunc;                    { M is a function that takes a Double (real) parameter

                                       and returns a Double }

procedure FuncProc(P: TIntegerFunction);  { FuncProc is a procedure whose only parameter

                                              is a parameterless integer-valued function }

The variables above are all procedure pointers—that is, pointers to the address of a procedure or function. If you want to reference a method of an instance object (see Classes and objects), you need to add the words of object to the procedural type name. For example

type

  TMethod = procedure of object;

  TNotifyEvent = procedure(Sender: TObject) of object;

These types represent method pointers. A method pointer is really a pair of pointers; the first stores the address of a method, and the second stores a reference to the object the method belongs to. Given the declarations

type

  TNotifyEvent = procedure(Sender: TObject) of object;

  TMainForm = class(TForm)

    procedure ButtonClick(Sender: TObject);

    ...

  end;

var

  MainForm: TMainForm;

  OnClick: TNotifyEvent

we could make the following assignment.

OnClick := MainForm.ButtonClick;

Two procedural types are compatible if they have

the same calling convention,

the same return value (or no return value), and

the same number of parameters, with identically typed parameters in corresponding positions. (Parameter names do not matter.)

Procedure pointer types are always incompatible with method pointer types. The value nil can be assigned to any procedural type.

Nested procedures and functions (routines declared within other routines) cannot be used as procedural values, nor can predefined procedures and functions. If you want to use a predefined routine like Length as a procedural value, write a wrapper for it:

function FLength(S: string): Integer;

begin

  Result := Length(S);

end;

Procedural types in statements and expressions


Procedural types in statements and expressions
  Object Pascal Language Guide

Procedural types in statements and expressions

Topic groups	See also

When a procedural variable is on the left side of an assignment statement, the compiler expects a procedural value on the right. The assignment makes the variable on the left a pointer to the function or procedure indicated on the right. In other contexts, however, using a procedural variable results in a call to the referenced procedure or function. You can even use a procedural variable to pass parameters:

var

  F: function(X: Integer): Integer;

  I: Integer;

function SomeFunction(X: Integer): Integer;

 ...

F := SomeFunction;  // assign SomeFunction to F

I := F(4);          // call function; assign result to I

In assignment statements, the type of the variable on the left determines the interpretation of procedure or method pointers on the right. For example,

var

  F, G: function: Integer;

  I: Integer;

function SomeFunction: Integer;

 ...

F := SomeFunction;  // assign SomeFunction to F

G := F;             // copy F to G

I := G;             // call function; assign result to I

The first statement assigns a procedural value to F. The second statement copies that value to another variable. The third statement makes a call to the referenced function and assigns the result to I. Because I is an integer variable, not a procedural one, the last assignment actually calls the function (which returns an integer).

In some situations it is less clear how a procedural variable should be interpreted. Consider the statement

if F = MyFunction then ...;

In this case, the occurrence of F results in a function call; the compiler calls the function pointed to by F, then calls the function MyFunction, then compares the results. The rule is that whenever a procedural variable occurs within an expression, it represents a call to the referenced procedure or function. In a case where F references a procedure (which doesn’t return a value), or where F references a function that requires parameters, the statement above causes a compilation error. To compare the procedural value of F with MyFunction, use

if @F = @MyFunction then ...;

@F converts F into an untyped pointer variable that contains an address, and @MyFunction returns the address of MyFunction.

To get the memory address of a procedural variable (rather than the address stored in it), use @@. For example, @@F returns the address of F.

The @ operator can also be used to assign an untyped pointer value to a procedural variable. For example,

var StrComp: function(Str1, Str2: PChar): Integer;

 ...

@StrComp := GetProcAddress(KernelHandle, 'lstrcmpi');

calls the GetProcAddress function and points StrComp to the result.

Any procedural variable can hold the value nil, which means that it points to nothing. But attempting to call a nil-valued procedural variable is an error. To test whether a procedural variable is assigned, use the standard function Assigned:

if Assigned(OnClick) then OnClick(X);


Variant types: Overview
  Object Pascal Language Guide

Variant types

Topic groups	See also

Sometimes it is necessary to manipulate data whose type varies or cannot be determined at compile time. In these cases, one option is to use variables and parameters of type Variant, which represent values that can change type at runtime. Variants offer greater flexibility but consume more memory than regular variables, and operations on them are slower than on statically bound types. Moreover, illicit operations on variants often result in runtime errors, where similar mistakes with regular variables would have been caught at compile time. You can also create custom variant types.

By default, Variants can hold values of any type except records, sets, static arrays, files, classes, class references, and pointers. In other words, variants can hold anything but structured types and pointers. They can hold interfaces, whose methods and properties can be accessed through them. (See Object interfaces.) They can hold dynamic arrays, and they can hold a special kind of static array called a variant array. (See Variant arrays.) Variants can mix with other variants and with integer, real, string, and Boolean values in expressions and assignments; the compiler automatically performs type conversions.

Variants that contain strings cannot be indexed. That is, if V is a variant that holds a string value, the construction V[1] causes a runtime error.

You can define custom Variants that extend the Variant type to hold arbitrary values. For example, you can define a Variant string type that allows indexing or that holds a particular class reference, record type, or static array. Custom Variant types are defined by creating descendants to the TCustomVariantType class.

A variant occupies 16 bytes of memory and consists of a type code and a value, or pointer to a value, of the type specified by the code. All variants are initialized on creation to the special value Unassigned. The special value Null indicates unknown or missing data.

The standard function VarType returns a variant’s type code. The varTypeMask constant is a bit mask used to extract the code from VarType’s return value, so that, for example,

VarType(V) and varTypeMask = varDouble

returns True if V contains a Double or an array of Double. (The mask simply hides the first bit, which indicates whether the variant holds an array.) The TVarData record type defined in the System unit can be used to typecast variants and gain access to their internal representation. See the online Help on VarType for a list if codes, and note that new type codes may be added in future implementations of Object Pascal.

Variant type conversions

Variants in expressions

Variant arrays

Other variant types


Variant type conversions
  Object Pascal Language Guide

Variant type conversions

Topic groups	See also

All integer, real, string, character, and Boolean types are assignment-compatible with Variant. Expressions can be explicitly cast as variants, and the VarAsType and VarCast standard routines can be used to change the internal representation of a variant. The following code demonstrates the use of variants and some of the automatic conversions performed when variants are mixed with other types.

var

  V1, V2, V3, V4, V5: Variant;

  I: Integer;

  D: Double;

  S: string;

begin

  V1 := 1; { integer value }

  V2 := 1234.5678; { real value }

  V3 := 'Hello world!'; { string value }

  V4 := '1000'; { string value }

  V5 := V1 + V2 + V4; { real value 2235.5678}

  I := V1; { I = 1 (integer value) }

  D := V2; { D = 1234.5678 (real value) }

  S := V3; { S = 'Hello world!' (string value) }

  I := V4; { I = 1000 (integer value) }

  S := V5; { S = '2235.5678' (string value) }

end;

The compiler performs type conversions according to the following rules.

Target Source	integer	real	string	character	Boolean
integer	converts integer formats	converts to real	converts to string representation	same as string (left)	returns False if 0, True otherwise
real	rounds to nearest integer	converts real formats	converts to string representation using regional settings	same as string (left)	returns False if 0, True otherwise
string	converts to integer, truncating if necessary; raises exception if string is not numeric	converts to real using regional settings; raises exception if string is not numeric	converts string/character formats	same as string (left)	returns False if string is “false” (non–case-sensitive) or a numeric string that evaluates to 0, True if string is “true” or a nonzero numeric string; raises exception otherwise
character	same as string (above)	same as string (above)	same as string (above)	same as string-to-string	same as string (above)
Boolean	False = 0,  True = –1  (255 if Byte)	False = 0,  True = –1	False = “0”,  True = “–1”	same as string (left)	False = False,  True = True
Unassigned	returns 0	returns 0	returns empty string	same as string (left)	returns False
Null	raises exception	raises exception	raises exception	same as string (left)	raises exception
Out-of-range assignments often result in the target variable getting the highest value in its range. Invalid assignments or casts raise the EVariantError exception.

Special conversion rules apply to the TDateTime real type declared in the System unit. When a TDateTime is converted to any other type, it treated as a normal Double. When an integer, real, or Boolean is converted to a TDateTime, it is first converted to a Double, then read as a date-time value. When a string is converted to a TDateTime, it is interpreted as a date-time value using the regional settings. When an Unassigned value is converted to TDateTime, it is treated like the real or integer value 0. Converting a Null value to TDateTime raises an exception.

On Windows, if a variant references a COM interface, any attempt to convert it reads the object’s default property and converts that value to the requested type. If the object has no default property, an exception is raised.


Variants in expressions
  Object Pascal Language Guide

Variants in expressions

Topic groups	See also

All operators except ^, is, and in take variant operands. Operations on variants return Variant values; they return Null if one or both operands is Null, and raise an exception if one or both operands is Unassigned. In a binary operation, if only one operand is a variant, the other is converted to a variant.

The return type of an operation is determined by its operands. In general, the same rules that apply to operands of statically bound types apply to variants. For example, if V1 and V2 are variants that hold an integer and a real value, then V1 + V2 returns a real-valued variant. With variants, however, you can perform binary operations on combinations of values that would not be allowed using statically typed expressions. When possible, the compiler converts mismatched variants using the rules summarized in Variant type conversions. For example, if V3 and V4 are variants that hold a numeric string and an integer, the expression V3 + V4 returns an integer-valued variant; the numeric string is converted to an integer before the operation is performed.


Variant arrays
  Object Pascal Language Guide

Variant arrays

Topic groups	See also

You cannot assign an ordinary static array to a variant. Instead, create a variant array by calling either of the standard functions VarArrayCreate or VarArrayOf. For example,

V: Variant;

 ...

V := VarArrayCreate([0,9], varInteger);

creates a variant array of integers (of length 10) and assigns it to the variant V. The array can be indexed using V[0], V[1], and so forth, but it is not possible to pass a variant array element as a var parameter. Variant arrays are always indexed with integers.

The second parameter in the call to VarArrayCreate is the type code for the array’s base type. For a list of these codes, see VarType. Never pass the code varString to VarArrayCreate; to create a variant array of strings, use varOleStr.

Variants can hold variant arrays of different sizes, dimensions, and base types. The elements of a variant array can be of any type allowed in variants except ShortString and AnsiString, and if the base type of the array is Variant, its elements can even be heterogeneous. Use the VarArrayRedim function to resize a variant array. Other standard routines that operate on variant arrays include VarArrayDimCount, VarArrayLowBound, VarArrayHighBound, VarArrayRef, VarArrayLock, and VarArrayUnlock.

When a variant containing a variant array is assigned to another variant or passed as a value parameter, the entire array is copied. Don’t perform such operations unnecessarily, since they are memory-inefficient.


OleVariant
  Object Pascal Language Guide

OleVariant

Topic groups	See also

The OleVariant type exists on both the Windows and Linux platforms. The main difference between Variant and OleVariant is that Variant can contain data types that only the current application knows what to do with. OleVariant can only contain the data types defined as compatible with Ole Automation which means that the data types that can be passed between programs or across the network without worrying about whether the other end will know how to handle the data.

When you assign a Variant that contains custom data (such as a Pascal string, or a one of the new custom variant types) to an OleVariant, the runtime library tries to convert the Variant into one of the OleVariant standard data types (such as a Pascal string converts to an Ole BSTR string). For example, if a variant containing an AnsiString is assigned to an OleVariant, the AnsiString becomes a WideString. The same is true when passing a Variant to an OleVariant function parameter.


Type compatibility and identity: Overview
  Object Pascal Language Guide

Type compatibility and identity

Topic groups	See also

To understand which operations can be performed on which expressions, we need to distinguish several kinds of compatibility among types and values. These include type identity, type compatibility, and assignment-compatibility.


Type identity
  Object Pascal Language Guide

Type identity

Topic groups	See also

Type identity is almost straightforward. When one type identifier is declared using another type identifier, without qualification, they denote the same type. Thus, given the declarations

type

  T1 = Integer;

  T2 = T1;

  T3 = Integer;

  T4 = T2;

T1, T2, T3, T4, and Integer all denote the same type. To create distinct types, repeat the word type in the declaration. For example,

type TMyInteger = type Integer;

creates a new type called TMyInteger which is not identical to Integer.

Language constructions that function as type names denote a different type each time they occur. Thus the declarations

type

  TS1 = set of Char;

  TS2 = set of Char;

create two distinct types, TS1 and TS2. Similarly, the variable declarations

var

  S1: string[10];

  S2: string[10];

create two variables of distinct types. To create variables of the same type, use

var S1, S2: string[10];

or

type MyString = string[10];

var

  S1: MyString;

  S2: MyString;


Type compatibility
  Object Pascal Language Guide

Type compatibility

Topic groups	See also

Every type is compatible with itself. Two distinct types are compatible if they satisfy at least one of the following conditions.

They are both real types.

They are both integer types.

One type is a subrange of the other.

Both types are subranges of the same type.

Both are set types with compatible base types.

Both are packed-string types with the same number of components.

One is a string type and the other is a string, packed-string, or Char type.

One type is Variant and the other is an integer, real, string, character, or Boolean type.

Both are class, class-reference, or interface types, and one type is derived from the other.

One type is PChar or PWideChar and the other is a zero-based character array of the form array[0..n] of Char.

One type is Pointer (an untyped pointer) and the other is any pointer type.

Both types are (typed) pointers to the same type and the {$T+} compiler directive is in effect.

Both are procedural types with the same result type, the same number of parameters, and type-identity between parameters in corresponding positions.


Assignment-compatibility
  Object Pascal Language Guide

Assignment-compatibility

Topic groups	See also

Assignment-compatibility is not a symmetric relation. An expression of type T2 can be assigned to a variable of type T1 if the value of the expression falls in the range of T1 and at least one of the following conditions is satisfied.

T1 and T2 are of the same type, and it is not a file type or structured type that contains a file type at any level.

T1 and T2 are compatible ordinal types.

T1 and T2 are both real types.

T1 is a real type and T2 is an integer type.

T1 is PChar or any string type and the expression is a string constant.

T1 and T2 are both string types.

T1 is a string type and T2 is a Char or packed-string type.

T1 is a long string and T2 is PChar.

T1 and T2 are compatible packed-string types.

T1 and T2 are compatible set types.

T1 and T2 are compatible pointer types.

T1 and T2 are both class, class-reference, or interface types and T2 is a derived from T1.

T1 is an interface type and T2 is a class type that implements T1.

T1 is PChar or PWideChar and T2 is a zero-based character array of the form array[0..n] of Char.

T1 and T2 are compatible procedural types. (A function or procedure identifier is treated, in certain assignment statements, as an expression of a procedural type. See “Procedural types in statements and expressions” on page 5-29.)

T1 is Variant and T2 is an integer, real, string, character, Boolean, or interface type.

T1 is an integer, real, string, character, or Boolean type and T2 is Variant.

T1 is the IUnknown or IDispatch interface type and T2 is Variant. (The variant’s type code must be varEmpty, varUnknown, or varDispatch if T1 is IUnknown, and varEmpty or varDispatch if T1 is IDispatch.)


Declaring types
  Object Pascal Language Guide

Declaring types

Topic groups	See also

A type declaration specifies an identifier that denotes a type. The syntax for a type declaration is

type newTypeName = type

where newTypeName is a valid identifier. For example, given the type declaration

type TMyString = string;

you can make the variable declaration

var S: TMyString;

A type identifier’s scope doesn’t include the type declaration itself (except for pointer types). So you cannot, for example, define a record type that uses itself recursively.

When you declare a type that is identical to an existing type, the compiler treats the new type identifier as an alias for the old one. Thus, given the declarations

type TValue = Real;

var

  X: Real;

  Y: TValue;

X and Y are of the same type; at runtime, there is no way to distinguish TValue from Real. This is usually of little consequence, but if your purpose in defining a new type is to utilize runtime type information—for example, to associate a property editor with properties of a particular type—the distinction between “different name” and “different type” becomes important. In this case, use the syntax

type newTypeName = type type

For example,

type TValue = type Real;

forces the compiler to create a new, distinct type called TValue.


Variables: Overview
  Object Pascal Language Guide

Variables

Topic groups	See also

A variable is an identifier whose value can change at runtime. Put differently, a variable is a name for a location in memory; you can use the name to read or write to the memory location. Variables are like containers for data, and, because they are typed, they tell the compiler how to interpret the data they hold.

Declaring variables


Declaring variables
  Object Pascal Language Guide

Declaring variables

Topic groups	See also

The basic syntax for a variable declaration is

var identifierList: type;

where identifierList is a comma-delimited list of valid identifiers and type is any valid type. For example,

var I: Integer;

declares a variable I of type Integer, while

var X, Y: Real;

declares two variables—X and Y—of type Real.

Consecutive variable declarations do not have to repeat the reserved word var:

var

  X, Y, Z: Double;

  I, J, K: Integer;

  Digit: 0..9;

  Okay: Boolean;

Variables declared within a procedure or function are sometimes called local, while other variables are called global. Global variables can be initialized at the same time they are declared, using the syntax

var identifier: type = constantExpression;

where constantExpression is any constant expression representing a value of type type. Thus the declaration

var I: Integer = 7;

is equivalent to the declaration and statement

var I: Integer;

 ...

I := 7;

Multiple variable declarations (such as var X, Y, Z: Real;) cannot include initializations, nor can declarations of variant and file-type variables.

If you don’t explicitly initialize a global variable, the compiler initializes it to 0. Local variables, in contrast, cannot be initialized in their declarations and contain random data until a value is assigned to them.

When you declare a variable, you are allocating memory which is freed automatically when the variable is no longer used. In particular, local variables exist only until the program exits from the function or procedure in which they are declared. For more information about variables and memory management, see Memory management.

Absolute addresses

Dynamic variables

Thread-local variables


Absolute addresses
  Object Pascal Language Guide

Absolute addresses

Topic groups	See also

You can create a new variable that resides at the same address as another variable. To do so, put the directive absolute after the type name in the declaration of the new variable, followed by the name of an existing (previously declared) variable. For example,

var

  Str: string[32];

  StrLen: Byte absolute Str;

specifies that the variable StrLen should start at the same address as Str. Since the first byte of a short string contains the string’s length, the value of StrLen is the length of Str.

You cannot initialize a variable in an absolute declaration or combine absolute with any other directives.


Dynamic variables
  Object Pascal Language Guide

Dynamic variables

Topic groups	See also

You can create dynamic variables by calling the GetMem or New procedure. Such variables are allocated on the heap and are not managed automatically. Once you create one, it is your responsibility ultimately to free the variable’s memory; use FreeMem to destroy variables created by GetMem and Dispose to destroy variables created by New. Other standard routines that operate on dynamic variables include ReallocMem, Initialize, StrAlloc, and StrDispose.

Long strings, wide strings, dynamic arrays, variants, and interfaces are also heap-allocated dynamic variables, but their memory is managed automatically.


Thread-local variables
  Object Pascal Language Guide

Thread-local variables

Topic groups	See also

Thread-local (or thread) variables are used in multithreaded applications. A thread-local variable is like a global variable, except that each thread of execution gets its own private copy of the variable, which cannot be accessed from other threads. Thread-local variables are declared with threadvar instead of var. For example,

threadvar X: Integer;

Thread-variable declarations

cannot occur within a procedure or function.

cannot include initializations.

cannot specify the absolute directive.

Do not create pointer- or procedural-type thread variables, and do not use thread variables in dynamically loadable libraries (other than packages).

Dynamic variables that are ordinarily managed by the compiler—long strings, wide strings, dynamic arrays, variants, and interfaces—can be declared with threadvar, but the compiler does not automatically free the heap-allocated memory created by each thread of execution. If you use these data types in thread variables, it is your responsibility to dispose of their memory. For example,

threadvar S: AnsiString;

S := 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

 ...

S := '';  // free the memory used by S

(You can free a variant by setting it to Unassigned and an interface or dynamic array by setting it to nil.)


Constants: Overview
  Object Pascal Language Guide

Declared constants

Topic groups	See also

Several different language constructions are referred to as “constants”. There are numeric constants (also called numerals) like 17, and string constants (also called character strings or string literals) like 'Hello world!'. Every enumerated type defines constants that represent the values of that type. There are predefined constants like True, False, and nil. Finally, there are constants that, like variables, are created individually by declaration.

Declared constants are either true constants or typed constants. These two kinds of constant are superficially similar, but they are governed by different rules and used for different purposes.


True constants
  Object Pascal Language Guide

True constants

Topic groups	See also

A true constant is a declared identifier whose value cannot change. For example,

const MaxValue = 237;

declares a constant called MaxValue that returns the integer 237. The syntax for declaring a true constant is

const identifier = constantExpression

where identifier is any valid identifier and constantExpression is an expression that the compiler can evaluate without executing your program. (See Constant expressions for more information.)

If constantExpression returns an ordinal value, you can specify the type of the declared constant using a value typecast. For example

const MyNumber = Int64(17);

declares a constant called MyNumber, of type Int64, that returns the integer 17. Otherwise, the type of the declared constant is the type of the constantExpression.

If constantExpression is a character string, the declared constant is compatible with any string type. If the character string is of length 1, it is also compatible with any character type.

If constantExpression is a real, its type is Extended. If it is an integer, its type is given by the table below.

Range of constant (hexadecimal)	Range of constant (decimal)	Type
–$8000000000000000..–$80000001	–2^63..–2147483649	Int64
–$80000000..–$8001	–2147483648..–32769	Integer
–$8000..–$81	–32768..–129	Smallint
–$80..–1	–128..–1	Shortint
0..$7F	0..127	0..127
$80..$FF	128..255	Byte
$0100..$7FFF	256..32767	0..32767
$8000..$FFFF	32768..65535	Word
$10000..$7FFFFFFF	65536..2147483647	0..2147483647
$80000000..$FFFFFFFF	2147483648..4294967295	Cardinal
$100000000..$7FFFFFFFFFFFFFFF	4294967296..2^63–1	Int64
Here are some examples of constant declarations:

const

  Min = 0;

  Max = 100;

  Center = (Max - Min) div 2;

  Beta = Chr(225);

  NumChars = Ord('Z') - Ord('A') + 1;

  Message = 'Out of memory';

  ErrStr = ' Error: ' + Message + '. ';

  ErrPos = 80 - Length(ErrStr) div 2;

  Ln10 = 2.302585092994045684;

  Ln10R = 1 / Ln10;

  Numeric = ['0'..'9'];

  Alpha = ['A'..'Z', 'a'..'z'];

  AlphaNum = Alpha + Numeric;


Constant expressions
  Object Pascal Language Guide

Constant expressions

Topic groups	See also

A constant expression is an expression that the compiler can evaluate without executing the program in which it occurs. Constant expressions include numerals; character strings; true constants; values of enumerated types; the special constants True, False, and nil; and expressions built exclusively from these elements with operators, typecasts, and set constructors. Constant expressions cannot include variables, pointers, or function calls, except calls to the following predefined functions:

Abs Chr Hi	High Length Lo	Low Odd Ord	Pred Round SizeOf	Succ Swap Trunc
This definition of a constant expression is used in several places in Object Pascal’s syntax specification. Constant expressions are required for initializing global variables, defining subrange types, assigning ordinalities to values in enumerated types, specifying default parameter values, writing case statements, and declaring both true and typed constants.

Examples of constant expressions:

100

'A'

256 - 1

(2.5 + 1) / (2.5 - 1)

'Borland' + ' ' + 'Developer'

Chr(32)

Ord('Z') - Ord('A') + 1


Resource strings
  Object Pascal Language Guide

Resource strings

Topic groups	See also

Resource strings are stored as resources and linked into the executable or library so that they can be modified without recompiling the program. For more information, see Internationalization and localization and the topics that follow it.

Resource strings are declared like other true constants, except that the word const is replaced by resourcestring. The expression to the right of the = symbol must be a constant expression and must return a string value. For example,

resourcestring

  CreateError = 'Cannot create file %s';        {  for explanations of format specifiers, }

  OpenError = 'Cannot open file %s';            { see 'Format strings' in the online Help }

  LineTooLong = 'Line too long';

  ProductName = 'Borland Rocks\000\000';

  SomeResourceString = SomeTrueConstant;

The compiler automatically resolves naming conflicts among resource strings in different libraries.


About typed constants
  Object Pascal Language Guide

Typed constants

Topic groups	See also

Typed constants, unlike true constants, can hold values of array, record, procedural, and pointer types. Typed constants cannot occur in constant expressions.

In the default {$J-} compiler state, typed constants can not have new values assigned to them; they are, in effect, read-only variables. However, if the {$J+} compiler directive is in effect, typed constants can have new values assigned to them; they behave essentially like initialized variables.

Declare a typed constant like this:

const identifier: type = value

where identifier is any valid identifier, type is any type except files and variants, and value is an expression of type type. For example,

const Max: Integer = 100;

In most cases, value must be a constant expression; but if type is an array, record, procedural, or pointer type, special rules apply.

Array constants

Record constants

Procedural constants

Pointer constants


Array constants
  Object Pascal Language Guide

Array constants

Topic groups	See also

To declare an array constant, enclose the values of the array’s elements, separated by commas, in parentheses at the end of the declaration. These values must be represented by constant expressions. For example,

const Digits: array[0..9] of Char = ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9');

declares a typed constant called Digits that holds an array of characters.

Zero-based character arrays often represent null-terminated strings, and for this reason string constants can be used to initialize character arrays. So the declaration above can be more conveniently represented as

const Digits: array[0..9] of Char = '0123456789';

To define a multidimensional array constant, enclose the values of each dimension in a separate set of parentheses, separated by commas. For example,

type TCube = array[0..1, 0..1, 0..1] of Integer;

const Maze: TCube = (((0, 1), (2, 3)), ((4, 5), (6,7)));

creates an array called Maze where

Maze[0,0,0] = 0

Maze[0,0,1] = 1

Maze[0,1,0] = 2

Maze[0,1,1] = 3

Maze[1,0,0] = 4

Maze[1,0,1] = 5

Maze[1,1,0] = 6

Maze[1,1,1] = 7

Array constants cannot contain file-type values at any level.


Record constants
  Object Pascal Language Guide

Record constants

Topic groups	See also

To declare a record constant, specify the value of each field—as fieldName: value, with the field assignments separated by semicolons—in parentheses at the end of the declaration. The values must be represented by constant expressions. The fields must be listed in the order in which they appear in the record type declaration, and the tag field, if there is one, must have a value specified; if the record has a variant part, only the variant selected by the tag field can be assigned values.

Examples:

type

  TPoint = record

    X, Y: Single;

  end;

  TVector = array[0..1] of TPoint;

  TMonth = (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec);

  TDate = record

    D: 1..31;

    M: TMonth;

    Y: 1900..1999;

  end;

const

  Origin: TPoint = (X: 0.0; Y: 0.0);

  Line: TVector = ((X: -3.1; Y: 1.5), (X: 5.8; Y: 3.0));

  SomeDay: TDate = (D: 2; M: Dec; Y: 1960);

Record constants cannot contain file-type values at any level.


Procedural constants
  Object Pascal Language Guide

Procedural constants

Topic groups	See also

To declare a procedural constant, specify the name of a function or procedure that is compatible with the declared type of the constant. For example,

function Calc(X, Y: Integer): Integer;

begin

 ...

end;

type TFunction = function(X, Y: Integer): Integer;

const MyFunction: TFunction = Calc;

Given these declarations, you can use the procedural constant MyFunction in a function call:

I := MyFunction(5, 7)

You can also assign the value nil to a procedural constant.


Pointer constants
  Object Pascal Language Guide

Pointer constants

Topic groups	See also

When you declare a pointer constant, you must initialize it to a value that can be resolved—at least as a relative address—at compile time. There are three ways to do this: with the @ operator, with nil, and (if the constant is of type PChar) with a string literal. For example, if I is a global variable of type Integer, you can declare a constant like

const PI: ^Integer = @I;

The compiler can resolve this because global variables are part of the code segment. So are functions and global constants:

const PF: Pointer = @MyFunction;

Because string literals are allocated as global constants, you can initialize a PChar constant with a string literal:

const WarningStr: PChar = 'Warning!';

Addresses of local (stack-allocated) and dynamic (heap-allocated) variables cannot be assigned to pointer constants.


Procedures and functions: Overview
  Object Pascal Language Guide

Procedures and functions

Topic groups	See also

Procedures and functions, referred to collectively as routines, are self-contained statement blocks that can be called from different locations in a program. A function is a routine that returns a value when it executes. A procedure is a routine that does not return a value.

Function calls, because they return a value, can be used as expressions in assignments and operations. For example,

I := SomeFunction(X);

calls SomeFunction and assigns the result to I. Function calls cannot appear on the left side of an assignment statement.

Procedure calls—and, when extended syntax is enabled ({$X+}), function calls—can be used as complete statements. For example,

DoSomething;

calls the DoSomething routine; if DoSomething is a function, its return value is discarded.

Procedures and functions can call themselves recursively.

Declaring procedures and functions

Parameters

Calling procedures and functions


Declaring procedures and functions: Overview
  Object Pascal Language Guide

Declaring procedures and functions

Topic groups	See also

When you declare a procedure or function, you specify its name, the number and type of parameters it takes, and, in the case of a function, the type of its return value; this part of the declaration is sometimes called the prototype, heading, or header. Then you write a block of code that executes whenever the procedure or function is called; this part is sometimes called the routine’s body or block.

The standard procedure Exit can occur within the body of any procedure or function. Exit halts execution of the routine where it occurs and immediately passes program control back to the point from which the routine was called.

Procedure declarations

Function declarations


Procedure declarations
  Object Pascal Language Guide

Procedure declarations

Topic groups	See also

A procedure declaration has the form

procedure procedureName(parameterList); directives;

localDeclarations;

begin

  statements

end;

where procedureName is any valid identifier, statements is a sequence of statements that execute when the procedure is called, and (parameterList), directives;, and localDeclarations; are optional.

For information about the parameterList, see Parameters.

For information about directives, see Calling conventions, Forward and interface declarations, External declarations, Overloading procedures and functions, and Writing dynamically loadable libraries. If you include more than one directive, separate them with semicolons.

For information about localDeclarations, which declares local identifiers, see Local declarations.

Here is an example of a procedure declaration:

procedure NumString(N: Integer; var S: string);

var

  V: Integer;

begin

  V := Abs(N);

  S := '';

  repeat

    S := Chr(V mod 10 + Ord('0')) + S;

    V := V div 10;

  until V = 0;

  if N < 0 then S := '-' + S;

end;

Given this declaration, you can call the NumString procedure like this:

NumString(17, MyString);

This procedure call assigns the value “17” to MyString (which must be a string variable).

Within a procedure’s statement block, you can use variables and other identifiers declared in the localDeclarations part of the procedure. You can also use the parameter names from the parameter list (like N and S in the example above); the parameter list defines a set of local variables, so don’t try to redeclare the parameter names in the localDeclarations section. Finally, you can use any identifiers within whose scope the procedure declaration falls.

Overloading procedures and functions


Function declarations
  Object Pascal Language Guide

Function declarations

Topic groups	See also

A function declaration is like a procedure declaration except that it specifies a return type and a return value. Function declarations have the form

function functionName(parameterList): returnType; directives;

localDeclarations;

begin

  statements

end;

where functionName is any valid identifier, returnType is any type, statements is a sequence of statements that execute when the function is called, and (parameterList), directives;, and localDeclarations; are optional.

For information about the parameterList, see Parameters.

For information about directives, see Calling conventions, Forward and interface declarations, External declarations, Overloading procedures and functions, and Writing dynamically loadable libraries. If you include more than one directive, separate them with semicolons.

For information about localDeclarations, which declares local identifiers, see Local declarations.

The function’s statement block is governed by the same rules that apply to procedures. Within the statement block, you can use variables and other identifiers declared in the localDeclarations part of the function, parameter names from the parameter list, and any identifiers within whose scope the function declaration falls. In addition, the function name itself acts as a special variable that holds the function’s return value, as does the predefined variable Result.

For example,

function WF: Integer;

begin

  WF := 17;

end;

defines a constant function called WF that takes no parameters and always returns the integer value 17. This declaration is equivalent to

function WF: Integer;

begin

  Result := 17;

end;

Here is a more complicated function declaration:

function Max(A: array of Real; N: Integer): Real;

var

  X: Real;

  I: Integer;

begin

  X := A[0];

  for I := 1 to N - 1 do

    if X < A[I] then X := A[I];

  Max := X;

end;

You can assign a value to Result or to the function name repeatedly within a statement block, as long as you assign only values that match the declared return type. When execution of the function terminates, whatever value was last assigned to Result or to the function name becomes the function’s return value. For example,

function Power(X: Real; Y: Integer): Real;

var

  I: Integer;

begin

  Result := 1.0;

  I := Y;

  while I > 0 do

  begin

    if Odd(I) then Result := Result * X;

    I := I div 2;

    X := Sqr(X);

  end;

end;

Result and the function name always represent the same value. Hence

function MyFunction: Integer;

begin

  MyFunction := 5;

  Result := Result * 2;

  MyFunction := Result + 1;

end;

returns the value 11. But Result is not completely interchangeable with the function name. When the function name appears on the left side of an assignment statement, the compiler assumes that it is being used (like Result) to track the return value; when the function name appears anywhere else in the statement block, the compiler interprets it as a recursive call to the function itself. Result, on the other hand, can be used as a variable in operations, typecasts, set constructors, indexes, and calls to other routines.

As long as extended syntax is enabled ({$X+}), Result is implicitly declared in every function. Do not try to redeclare it.

If execution terminates without an assignment being made to Result or the function name, then the function’s return value is undefined.

Overloading procedures and functions


Calling conventions
  Object Pascal Language Guide

Calling conventions

Topic groups	See also

When you declare a procedure or function, you can specify a calling convention using one of the directives register, pascal, cdecl, stdcall, and safecall. For example,

function MyFunction(X, Y: Real): Real; cdecl;

 ...

Calling conventions determine the order in which parameters are passed to the routine. They also affect the removal of parameters from the stack, the use of registers for passing parameters, and error and exception handling. The default calling convention is register.

The register and pascal conventions pass parameters from left to right; that is, the leftmost parameter is evaluated and passed first and the rightmost parameter is evaluated and passed last. The cdecl, stdcall, and safecall conventions pass parameters from right to left.

For all conventions except cdecl, the procedure or function removes parameters from the stack upon returning. With the cdecl convention, the caller removes parameters from the stack when the call returns.

The register convention uses up to three CPU registers to pass parameters, while the other conventions pass all parameters on the stack.

The safecall convention implements exception “firewalls.” On Windows, this implements interprocess COM error notification.

The table below summarizes calling conventions.

Directive	Parameter order	Clean-up	Passes parameters in registers?
register	Left-to-right	Routine	Yes
pascal	Left-to-right	Routine	No
cdecl	Right-to-left	Caller	No
stdcall	Right-to-left	Routine	No
safecall	Right-to-left	Routine	No
The default register convention is the most efficient, since it usually avoids creation of a stack frame. (Access methods for published properties must use register.) The cdecl convention is useful when you call functions from shared libraries written in C or C++, while stdcall and safecall are recommended, in general, for calls to external code. On Windows, the operating system APIs are stdcall and safecall. Other operating systems generally use cdecl. (Note that stdcall is more efficient than cdecl.)

The safecall convention must be used for declaring dual-interface methods. The pascal convention is maintained for backward compatibility. For more information on calling conventions, see Program control.

The directives near, far, and export refer to calling conventions in 16-bit Windows programming. They have no effect in 32-bit applications and are maintained for backward compatibility only.


Forward and interface declarations
  Object Pascal Language Guide

Forward and interface declarations

Topic groups	See also

The forward directive replaces the block, including local variable declarations and statements, in a procedure or function declaration. For example,

function Calculate(X, Y: Integer): Real; forward;

declares a function called Calculate. Somewhere after the forward declaration, the routine must be redeclared in a defining declaration that includes a block. The defining declaration for Calculate might look like this:

function Calculate;

 ...   { declarations }

begin

 ...   { statement block }

end;

Ordinarily, a defining declaration does not have to repeat the routine’s parameter list or return type, but if it does repeat them, they must match those in the forward declaration exactly (except that default parameters can be omitted). If the forward declaration specifies an overloaded procedure or function (see Overloading procedures and functions), then the defining declaration must repeat the parameter list.

Between a forward declaration and its defining declaration, you can place nothing except other declarations. The defining declaration can be an external or assembler declaration, but it cannot be another forward declaration.

The purpose of a forward declaration is to extend the scope of a procedure or function identifier to an earlier point in the source code. This allows other procedures and functions to call the forward-declared routine before it is actually defined. Besides letting you organize your code more flexibly, forward declarations are sometimes necessary for mutual recursions.

The forward directive is not allowed in the interface section of a unit. Procedure and function headers in the interface section, however, behave like forward declarations and must have defining declarations in the implementation section. A routine declared in the interface section is available from anywhere else in the unit and from any other unit or program that uses the unit where it is declared.


External declarations
  Object Pascal Language Guide

External declarations

Topic groups	See also

The external directive, which replaces the block in a procedure or function declaration, allows you to call routines that are compiled separately from your program. External routines can come from object files or dynamically loadable libraries.

When importing a C++ function that takes a variable number of parameters, use the varargs directive. For example,

function printf(Format: PChar): Integer; cdecl; varargs;

The varargs directive works only with external routines and only with the cdecl calling convention.

Linking to object files

To call routines from a separately compiled object file, first link the object file to your application using the $L (or $LINK) compiler directive. For example,

On Windows: {$L BLOCK.OBJ}

On Linux:   {$L block.o}

links BLOCK.OBJ (Windows) or block.o (Linux) into the program or unit in which it occurs. Next, declare the functions and procedures that you want to call:

procedure MoveWord(var Source, Dest; Count: Integer); external;

procedure FillWord(var Dest; Data: Integer; Count: Integer); external;

Now you can call the MoveWord and FillWord routines from BLOCK.OBJ (Windows) or block.o (Linux).

Declarations like the ones above are frequently used to access external routines written in assembly language. You can also place assembly-language routines directly in your Object Pascal source code; for more information, see Inline assembler code.

Importing functions from libraries

To import routines from a dynamically loadable library (.so or .DLL), attach a directive of the form

external stringConstant;

to the end of a normal procedure or function header, where stringConstant is the name of the library file in single quotation marks. For example, on Windows

function SomeFunction(S: string): string; external 'strlib.dll';

imports a function called SomeFunction from strlib.dll.

On Linux,

function SomeFunction(S: string): string; external 'strlib.so';

imports a function called SomeFunction from strlib.so.

You can import a routine under a different name from the one it has in the library. If you do this, specify the original name in the external directive:

external stringConstant1 name stringConstant2;

where the first stringConstant gives the name of the library file and the second stringConstant is the routine’s original name.

On Windows: For example, the following declaration imports a function from user32.dll (part of the Windows API).

function MessageBox(HWnd: Integer; Text, Caption: PChar; Flags: Integer): Integer;
  stdcall; external 'user32.dll' name 'MessageBoxA';

The function’s original name is MessageBoxA, but it is imported as MessageBox.

Instead of a name, you can use a number to identify the routine you want to import:

external stringConstant index integerConstant;

where integerConstant is the routine’s index in the export table.

On Linux: For example, the following declaration imports a standard system function from libc.so.6.

function OpenFile(const PathName: PChar; Flags: Integer): Integer; cdecl;
  external 'libc.so.6' name 'open';

The function’s original name is open, but it is imported as OpenFile.

In your importing declaration, be sure to match the exact spelling and case of the routine’s name. Later, when you call the imported routine, the name is case-insensitive.

For more information about libraries, see Libraries and packages.


Overloading procedures and functions
  Object Pascal Language Guide

Overloading procedures and functions

Topic groups	See also

You can declare more than one routine in the same scope with the same name. This is called overloading. Overloaded routines must be declared with the overload directive and must have distinguishing parameter lists. For example, consider the declarations

function Divide(X, Y: Real): Real; overload;

begin

  Result := X/Y;

end;

function Divide(X, Y: Integer): Integer; overload;

begin

  Result := X div Y;

end;

These declarations create two functions, both called Divide, that take parameters of different types. When you call Divide, the compiler determines which function to invoke by looking at the actual parameters passed in the call. For example, Divide(6.0, 3.0) calls the first Divide function, because its arguments are real-valued.

You can pass to an overloaded routine parameters that are not identical in type with those in any of the routine’s declarations, but that are assignment-compatible with the parameters in more than one declaration. This happens most frequently when a routine is overloaded with different integer types or different real types—for example,

procedure Store(X: Longint); overload;

procedure Store(X: Shortint); overload;

In these cases, when it is possible to do so without ambiguity, the compiler invokes the routine whose parameters are of the type with the smallest range that accommodates the actual parameters in the call. (Remember that real-valued constant expressions are always of type Extended.)

Overloaded routines must be distinguished by the number of parameters they take or the types of their parameters. Hence the following pair of declarations causes a compilation error.

function Cap(S: string): string; overload;

 ...

procedure Cap(var Str: string); overload;

 ...

But the declarations

function Func(X: Real; Y: Integer): Real; overload;

 ...

function Func(X: Integer; Y: Real): Real; overload;

 ...

are legal.

When an overloaded routine is declared in a forward or interface declaration, the defining declaration must repeat the routine’s parameter list.

The compiler can distinguish between overloaded functions that contain AnsiString/PChar and WideString/WideChar parameters in the same parameter position. String constants or literals passed into such an overload situation are translated into the native string or character type, which is AnsiString/PChar.

  procedure test(const S: String);  overload;

  procedure test(const W: WideString); overload;

  var

      a: string;

      b: widestring;

  begin

     a := 'a';

     b := 'b';

     test(a);    // calls String version

     test(b);    // calls WideString version

     test('abc');    // calls String version

     test(WideString('abc'));   // calls widestring version

  end;

Variants can also be used as parameters in overloaded function declarations. Variant is considered more general than any simple type. Preference is always given to exact type matches over variant matches. If a variant is passed into such an overload situation, and an overload that takes a variant exists in that parameter position, it is considered to be an exact match for the Variant type.

This can cause some minor side effects with float types. Float types are matched by size. If there is no exact match for the float variable passed to the overload call but a variant parameter is available, the variant is taken over any smaller float type.

For example:

  procedure foo(i: integer); overload;

  procedure foo(d: double);  overload;

  procedure foo(v: variant); overload;

  var

    v: variant;

  begin

    foo(1);       // integer version

    foo(v);       // variant version

    foo(1.2);     // variant version (float literals -> extended precision)

  end;

This example calls the variant version of foo, not the double version, because the 1.2 constant is implicitly an extended type and extended is not an exact match for double. Extended is also not an exact match for variant, but variant is considered a more general type (whereas double is a smaller type than extended).

foo(Double(1.2));

This typecast does not work. You should use typed consts instead.

const  d: double = 1.2;

  begin

     foo(d);

  end;

The above code works correctly, and calls the double version.

  const  s: single = 1.2;

  begin

     foo(s);

  end;

The above code also calls the double version of foo. Single is a better fit to double than to variant.

When declaring a set of overloaded routines, the best way to avoid float promotion to variant is to declare a version of your overloaded function for each float type (Single, Double, Extended) along with the variant version.

If you use default parameters in overloaded routines, be careful of ambiguous parameter signatures. For more information, see Default parameters and overloaded routines.

You can limit the potential effects of overloading by qualifying a routine’s name when you call it. For example, Unit1.MyProcedure(X, Y) can call only routines declared in Unit1; if no routine in Unit1 matches the name and parameter list in the call, an error results.

For information about distributing overloaded methods in a class hierarchy, see Overloading methods. For information about exporting overloaded routines from a shared library, see The exports clause.


Local declarations
  Object Pascal Language Guide

Local declarations

Topic groups	See also

The body of a function or procedure often begins with declarations of local variables used in the routine’s statement block. These declarations can also include constants, types, and other routines. The scope of a local identifier is limited to the routine where it is declared.

Nested routines

Functions and procedures sometimes contain other functions and procedures within the local-declarations section of their blocks. For example, the following declaration of a procedure called DoSomething contains a nested procedure.

procedure DoSomething(S: string);

var

  X, Y: Integer;

  procedure NestedProc(S: string);

  begin

    ...

  end;

begin

   ...

  NestedProc(S);

   ...

end;

The scope of a nested routine is limited to the procedure or function in which it is declared. In our example, NestedProc can be called only within DoSomething.

For real examples of nested routines, look at the DateTimeToString procedure, the ScanDate function, and other routines in the SysUtils unit.


Parameters: Overview
  Object Pascal Language Guide

Parameters

Topic groups	See also

Most procedure and function headers include a parameter list. For example, in the header

function Power(X: Real; Y: Integer): Real;

the parameter list is (X: Real; Y: Integer).

A parameter list is a sequence of parameter declarations separated by semicolons and enclosed in parentheses. Each declaration is a comma-delimited series of parameter names, followed in most cases by a colon and a type identifier, and in some cases by the = symbol and a default value. Parameter names must be valid identifiers. Any declaration can be preceded by one of the reserved words var, const, and out (see Parameter semantics). Examples:

(X, Y: Real)

(var S: string; X: Integer)

(HWnd: Integer; Text, Caption: PChar; Flags: Integer)

(const P; I: Integer)

The parameter list specifies the number, order, and type of parameters that must be passed to the routine when it is called. If a routine does not take any parameters, omit the identifier list and the parentheses in its declaration:

procedure UpdateRecords;

begin

 ...

end;

Within the procedure or function body, the parameter names (X and Y in the first example above) can be used as local variables. Do not redeclare the parameter names in the local declarations section of the procedure or function body.

String parameters

Array parameters


Parameter semantics: Overview
  Object Pascal Language Guide

Parameter semantics

Topic groups	See also

Parameters are categorized in several ways:

Every parameter is classified as value, variable, constant, or out. Value parameters are the default; the reserved words var, const, and out indicate variable, constant, and out parameters, respectively.

Value parameters are always typed, while constant, variable, and out parameters can be either typed or untyped.

Special rules apply to array parameters.

Files and instances of structured types that contain files can be passed only as variable (var) parameters.

Value and variable parameters

Constant parameters

Out parameters


Value and variable parameters
  Object Pascal Language Guide

Value and variable parameters

Topic groups	See also

Most parameters are either value parameters (the default) or variable (var) parameters. Value parameters are passed by value, while variable parameters are passed by reference. To see what this means, consider the following functions.

function DoubleByValue(X: Integer): Integer;     // X is a value parameter

begin

  X := X * 2;

  Result := X;

end;

function DoubleByRef(var X: Integer): Integer;   // X is a variable parameter

begin

  X := X * 2;

  Result := X;

end;

These functions return the same result, but only the second one—DoubleByRef—can change the value of a variable passed to it. Suppose we call the functions like this:

var

  I, J, V, W: Integer;

begin

  I := 4;

  V := 4;

  J := DoubleByValue(I);   // J = 8, I = 4

  W := DoubleByRef(V);     // W = 8, V = 8

end;

After this code executes, the variable I, which was passed to DoubleByValue, has the same value we initially assigned to it. But the variable V, which was passed to DoubleByRef, has a different value.

A value parameter acts like a local variable that gets initialized to the value passed in the procedure or function call. If you pass a variable as a value parameter, the procedure or function creates a copy of it; changes made to the copy have no effect on the original variable and are lost when program execution returns to the caller.

A variable parameter, on the other hand, acts like a pointer rather than a copy. Changes made to the parameter within the body of a function or procedure persist after program execution returns to the caller and the parameter name itself has gone out of scope.

Even if the same variable is passed in two or more var parameters, no copies are made. This is illustrated in the following example.

procedure AddOne(var X, Y: Integer);

begin

  X := X + 1;

  Y := Y + 1;

end;

var I: Integer;

begin

  I := 1;

  AddOne(I, I);

end;

After this code executes, the value of I is 3.

If a routine’s declaration specifies a var parameter, you must pass an assignable expression—that is, a variable, typed constant (in the {$J+} state), dereferenced pointer, field, or indexed variable—to the routine when you call it. To use our previous examples, DoubleByRef(7) produces an error, although DoubleByValue(7) is legal.

Indexes and pointer dereferences passed in var parameters—for example, DoubleByRef(MyArray[I])—are evaluated once, before execution of the routine.


Constant parameters
  Object Pascal Language Guide

Constant parameters

Topic groups	See also

A constant (const) parameter is like a local constant or read-only variable. Constant parameters are similar to value parameters, except that you can’t assign a value to a constant parameter within the body of a procedure or function, nor can you pass one as a var parameter to another routine. (But when you pass an object reference as a constant parameter, you can still modify the object’s properties.)

Using const allows the compiler to optimize code for structured- and string-type parameters. It also provides a safeguard against unintentionally passing a parameter by reference to another routine.

Here, for example, is the header for the CompareStr function in the SysUtils unit:

function CompareStr(const S1, S2: string): Integer;

Because S1 and S2 are not modified in the body of CompareStr, they can be declared as constant parameters.


Out parameters
  Object Pascal Language Guide

Out parameters

Topic groups	See also

An out parameter, like a variable parameter, is passed by reference. With an out parameter, however, the initial value of the referenced variable is discarded by the routine it is passed to. The out parameter is for output only; that is, it tells the function or procedure where to store output, but doesn’t provide any input.

For example, consider the procedure heading

procedure GetInfo(out Info: SomeRecordType);

When you call GetInfo, you must pass it a variable of type SomeRecordType:

var MyRecord: SomeRecordType;

 ...

GetInfo(MyRecord);

But you’re not using MyRecord to pass any data to the GetInfo procedure; MyRecord is just a container where you want GetInfo to store the information it generates. The call to GetInfo immediately frees the memory used by MyRecord, before program control passes to the procedure.

Out parameters are frequently used with distributed-object models like COM and CORBA. In addition, you should use out parameters when you pass an uninitialized variable to a function or procedure.


Untyped parameters
  Object Pascal Language Guide

Untyped parameters

Topic groups	See also

You can omit type specifications when declaring var, const, and out parameters. (Value parameters must be typed.) For example,

procedure TakeAnything(const C);

declares a procedure called TakeAnything that accepts a parameter of any type. When you call such a routine, you cannot pass it a numeral or untyped numeric constant.

Within a procedure or function body, untyped parameters are incompatible with every type. To operate on an untyped parameter, you must cast it. In general, the compiler cannot verify that operations on untyped parameters are valid.

The following example uses untyped parameters in a function called Equal that compares a specified number of bytes of any two variables.

function Equal(var Source, Dest; Size: Integer): Boolean;

type

  TBytes = array[0..MaxInt - 1] of Byte;

var

  N: Integer;

begin

  N := 0;

  while (N < Size) and (TBytes(Dest)[N] = TBytes(Source)[N]) do

    Inc(N);

  Equal := N = Size;

end;

Given the declarations

type

  TVector = array[1..10] of Integer;

  TPoint = record

    X, Y: Integer;

  end;

var

  Vec1, Vec2: TVector;

  N: Integer;

  P: TPoint;

you could make the following calls to Equal:

Equal(Vec1, Vec2, SizeOf(TVector))             // compare Vec1 to Vec2

Equal(Vec1, Vec2, SizeOf(Integer) * N)         // compare first N elements of Vec1 and Vec2

Equal(Vec1[1], Vec1[6], SizeOf(Integer) * 5)   // compare first 5 to last 5 elements of Vec1

Equal(Vec1[1], P, 4)                           // compare Vec1[1] to P.X and Vec1[2] to P.Y


About string parameters
  Object Pascal Language Guide

String parameters

Topic groups	See also

When you declare routines that take short-string parameters, you cannot include length specifiers in the parameter declarations. That is, the declaration

procedure Check(S: string[20]);   // syntax error

causes a compilation error. But

type TString20 = string[20];

procedure Check(S: TString20);

is valid. The special identifier OpenString can be used to declare routines that take short-string parameters of varying length:

procedure Check(S: OpenString);

When the {$H–} and {$P+} compiler directives are both in effect, the reserved word string is equivalent to OpenString in parameter declarations.

Short strings, OpenString, $H, and $P are supported for backward compatibility only. In new code, you can avoid these considerations by using long strings.


Array parameters: Overview
  Object Pascal Language Guide

Array parameters

Topic groups	See also

When you declare routines that take array parameters, you cannot include index type specifiers in the parameter declarations. That is, the declaration

procedure Sort(A: array[1..10] of Integer);   // syntax error

causes a compilation error. But

type TDigits = array[1..10] of Integer;

procedure Sort(A: TDigits);

is valid. For most purposes, however, open array parameters are a better solution.


Open array parameters
  Object Pascal Language Guide

Open array parameters

Topic groups	See also

Open array parameters allow arrays of different sizes to be passed to the same procedure or function. To define a routine with an open array parameter, use the syntax array of type (rather than array[X..Y] of type) in the parameter declaration. For example,

function Find(A: array of Char): Integer;

declares a function called Find that takes a character array of any size and returns an integer.

Note:	The syntax of open array parameters resembles that of dynamic array types, but they do not mean the same thing. The example above creates a function that takes any array of Char elements, including (but not limited to) dynamic arrays. To declare parameters that must be dynamic arrays, you need to specify a type identifier:

type TDynamicCharArray = array of Char;

function Find(A: TDynamicCharArray): Integer;

For information about dynamic arrays, see Dynamic arrays.

Within the body of a routine, open array parameters are governed by the following rules.

They are always zero-based. The first element is 0, the second element is 1, and so forth. The standard Low and High functions return 0 and Length – 1, respectively. The SizeOf function returns the size of the actual array passed to the routine.

They can be accessed by element only. Assignments to an entire open array parameter are not allowed.

They can be passed to other procedures and functions only as open array parameters or untyped var parameters. They cannot be passed to SetLength.

Instead of an array, you can pass a variable of the open array parameter’s base type. It will be treated as an array of length 1.

When you pass an array as an open array value parameter, the compiler creates a local copy of the array within the routine’s stack frame. Be careful not to overflow the stack by passing large arrays.

The following examples use open array parameters to define a Clear procedure that assigns zero to each element in an array of reals and a Sum function that computes the sum of the elements in an array of reals.

procedure Clear(var A: array of Real);

var

  I: Integer;

begin

  for I := 0 to High(A) do A[I] := 0;

end;

function Sum(const A: array of Real): Real;

var

  I: Integer;

  S: Real;

begin

  S := 0;

  for I := 0 to High(A) do S := S + A[I];

  Sum := S;

end;

When you call routines that use open array parameters, you can pass open array constructors to them.

Variant open array parameters


Variant open array parameters
  Object Pascal Language Guide

Variant open array parameters

Topic groups	See also

Variant open array parameters allow you to pass an array of differently-typed expressions to a single procedure or function. To define a routine with a variant open array parameter, specify array of const as the parameter’s type. Thus

procedure DoSomething(A: array of const);

declares a procedure called DoSomething that can operate on heterogeneous arrays.

The array of const construction is equivalent to array of TVarRec. TVarRec, declared in the System unit, represents a record with a variant part that can hold values of integer, Boolean, character, real, string, pointer, class, class reference, interface, and variant types. TVarRec’s VType field indicates the type of each element in the array. Some types are passed as pointers rather than values; in particular, long strings are passed as Pointer and must be typecast to string.

The following example uses a variant open array parameter in a function that creates a string representation of each element passed to it and concatenates the results into a single string. The string-handling routines called in this function are defined in SysUtils.

function MakeStr(const Args: array of const): string;

const

  BoolChars: array[Boolean] of Char = ('F', 'T');

var

  I: Integer;

begin

  Result := '';

  for I := 0 to High(Args) do

    with Args[I] do

      case VType of

        vtInteger:    Result := Result + IntToStr(VInteger);

        vtBoolean:    Result := Result + BoolChars[VBoolean];

        vtChar:       Result := Result + VChar;

        vtExtended:   Result := Result + FloatToStr(VExtended^);

        vtString:     Result := Result + VString^;

        vtPChar:      Result := Result + VPChar;

        vtObject:     Result := Result + VObject.ClassName;

        vtClass:      Result := Result + VClass.ClassName;

        vtAnsiString: Result := Result + string(VAnsiString);

        vtCurrency:   Result := Result + CurrToStr(VCurrency^);

        vtVariant:    Result := Result + string(VVariant^);

        vtInt64:      Result := Result + IntToStr(VInt64^);

    end;

end;

We can call this function using an open array constructor (see Open array constructors). For example,

MakeStr(['test', 100, ' ', True, 3.14159, TForm])

returns the string “test100 T3.14159TForm”.


Default parameters
  Object Pascal Language Guide

Default parameters

Topic groups	See also

You can specify default parameter values in a procedure or function heading. Default values are allowed only for typed const and value parameters. To provide a default value, end the parameter declaration with the = symbol followed by a constant expression that is assignment-compatible with the parameter’s type.

For example, given the declaration

procedure FillArray(A: array of Integer; Value: Integer = 0);

the following procedure calls are equivalent.

FillArray(MyArray);

FillArray(MyArray, 0);

A multiple-parameter declaration cannot specify a default value. Thus, while

function MyFunction(X: Real = 3.5; Y: Real = 3.5): Real;

is legal,

function MyFunction(X, Y: Real = 3.5): Real;   // syntax error

is not.

Parameters with default values must occur at the end of the parameter list. That is, all parameters following the first declared default value must also have default values. So the following declaration is illegal.

procedure MyProcedure(I: Integer = 1; S: string);   // syntax error

Default values specified in a procedural type override those specified in an actual routine. Thus, given the declarations

type TResizer = function(X: Real; Y: Real = 1.0): Real;

function Resizer(X: Real; Y: Real = 2.0): Real;

var

  F: TResizer;

  N: Real;

the statements

F := Resizer;

F(N);

result in the values (N, 1.0) being passed to Resizer.

Default parameters are limited to values that can be specified by a constant expression. Hence parameters of a dynamic-array, procedural, class, class-reference, or interface type can have no value other than nil as their default. Parameters of a record, variant, file, static-array, or object type cannot have default values at all.

For information about calling routines with default parameter values, see Calling procedures and functions.

Default parameters and overloaded routines

Default parameters in forward and interface declarations


Default parameters and overloaded routines
  Object Pascal Language Guide

Default parameters and overloaded routines

Topic groups	See also

If you use default parameter values in an overloaded routine, avoid ambiguous parameter signatures. Consider, for example, the following.

procedure Confused(I: Integer); overload;

 ...

procedure Confused(I: Integer; J: Integer = 0); overload;

 ...

Confused(X);    //  Which procedure is called?

In fact, neither procedure is called. This code generates a compilation error.


Default parameters in forward and interface declarations
  Object Pascal Language Guide

Default parameters in forward and interface declarations

Topic groups	See also

If a routine has a forward declaration or appears in the interface section of a unit, default parameter values—if there are any—must be specified in the forward or interface declaration. In this case, the default values can be omitted from the defining (implementation) declaration; but if the defining declaration includes default values, they must match those in the forward or interface declaration exactly.


Calling procedures and functions
  Object Pascal Language Guide

Calling procedures and functions

Topic groups	See also

When you call a procedure or function, program control passes from the point where the call is made to the body of the routine. You can make the call using the routine’s declared name (with or without qualifiers) or using a procedural variable that points to the routine. In either case, if the routine is declared with parameters, your call to it must pass parameters that correspond in order and type to the routine’s parameter list. The parameters you pass to a routine are called actual parameters, while the parameters in the routine’s declaration are called formal parameters.

When calling a routine, remember that

expressions used to pass typed const and value parameters must be assignment-compatible with the corresponding formal parameters.

expressions used to pass var and out parameters must be identically typed with the corresponding formal parameters, unless the formal parameters are untyped.

only assignable expressions can be used to pass var and out parameters.

if a routine’s formal parameters are untyped, numerals and true constants with numeric values cannot be used as actual parameters.

When you call a routine that uses default parameter values, all actual parameters following the first accepted default must also use the default values; calls of the form SomeFunction(,,X) are not legal.

You can omit parentheses when passing all and only the default parameters to a routine. For example, given the procedure

procedure DoSomething(X: Real = 1.0; I: Integer = 0; S: string = '');

the following calls are equivalent.

DoSomething();

DoSomething;

Open array constructors


Open array constructors
  Object Pascal Language Guide

Open array constructors

Topic groups	See also

Open array constructors allow you to construct arrays directly within function and procedure calls. They can be passed only as open array parameters or variant open array parameters.

An open array constructor, like a set constructor, is a sequence of expressions separated by commas and enclosed in brackets.

For example, given the declarations

var I, J: Integer;

procedure Add(A: array of Integer);

you could call the Add procedure with the statement

Add([5, 7, I, I + J]);

This is equivalent to

var Temp: array[0..3] of Integer;

 ...

Temp[0] := 5;

Temp[1] := 7;

Temp[2] := I;

Temp[3] := I + J;

Add(Temp);

Open array constructors can be passed only as value or const parameters. The expressions in a constructor must be assignment-compatible with the base type of the array parameter. In the case of a variant open array parameter, the expressions can be of different types.


Classes and objects: Overview
  Object Pascal Language Guide

Classes and objects

Topic groups	See also

A class, or class type, defines a structure consisting of fields, methods, and properties. Instances of a class type are called objects. The fields, methods, and properties of a class are called its components or members.

A field is essentially a variable that is part of an object. Like the fields of a record, a class’s fields represent data items that exist in each instance of the class.

A method is a procedure or function associated with a class. Most methods operate on objects—that is, instances of a class. Some methods (called class methods) operate on class types themselves.

A property is an interface to data associated with an object (often stored in a field). Properties have access specifiers, which determine how their data are read and modified. From other parts of a program—outside of the object itself—a property appears in most respects like a field.

Objects are dynamically allocated blocks of memory whose structure is determined by their class type. Each object has a unique copy of every field defined in the class, but all instances of a class share the same methods. Objects are created and destroyed by special methods called constructors and destructors.

A variable of a class type is actually a pointer that references an object. Hence more than one variable can refer to the same object. Like other pointers, class-type variables can hold the value nil. But you don’t have to explicitly dereference a class-type variable to access the object it points to. For example, SomeObject.Size := 100 assigns the value 100 to the Size property of the object referenced by SomeObject; you would not write this as SomeObject^.Size := 100.


About class types
  Object Pascal Language Guide

About class types

Topic groups	See also

A class type must be declared and given a name before it can be instantiated. (You cannot define a class type within a variable declaration.) Declare classes only in the outermost scope of a program or unit, not in a procedure or function declaration.

A class type declaration has the form

type className = class (ancestorClass)

  memberList

 end;

where className is any valid identifier, (ancestorClass) is optional, and memberList declares members—that is, fields, methods, and properties—of the class. If you omit (ancestorClass), then the new class inherits directly from the predefined TObject class. If you include (ancestorClass) and memberList is empty, you can omit end. A class type declaration can also include a list of interfaces implemented by the class; see Implementing interfaces.

Methods appear in a class declaration as function or procedure headings, with no body. Defining declarations for each method occur elsewhere in the program.

For example, here is the declaration of the TMemoryStream class from the Classes unit.

type

TMemoryStream = class(TCustomMemoryStream)

  private

    FCapacity: Longint;

    procedure SetCapacity(NewCapacity: Longint);

  protected

    function Realloc(var NewCapacity: Longint): Pointer; virtual;

    property Capacity: Longint read FCapacity write SetCapacity;

  public

    destructor Destroy; override;

    procedure Clear;

    procedure LoadFromStream(Stream: TStream);

    procedure LoadFromFile(const FileName: string);

    procedure SetSize(NewSize: Longint); override;

    function Write(const Buffer; Count: Longint): Longint; override;

  end;

TMemoryStream descends from TStream (in the Classes unit), inheriting most of its members. But it defines—or redefines—several methods and properties, including its destructor method, Destroy. Its constructor, Create, is inherited without change from TObject, and so is not redeclared. Each member is declared as private, protected, or public (this class has no published members); for explanations of these terms, see Visibility of class members.

Given this declaration, you can create an instance of TMemoryStream as follows:

var stream: TMemoryStream;

stream := TMemoryStream.Create;

Inheritance and scope

Forward declarations and mutually dependent classes


Inheritance and scope
  Object Pascal Language Guide

Inheritance and scope

Topic groups	See also

When you declare a class, you can specify its immediate ancestor. For example,

type TSomeControl = class(TControl);

declares a class called TSomeControl that descends from TControl. A class type automatically inherits all of the members from its immediate ancestor. Each class can declare new members and can redefine inherited ones, but a class cannot remove members defined in an ancestor. Hence TSomeControl contains all of the members defined in TControl and in each of TControl‘s ancestors.

The scope of a member’s identifier starts at the point where the member is declared, continues to the end of the class declaration, and extends over all descendants of the class and the blocks of all methods defined in the class and its descendants.

TObject and TClass

Compatibility of class types


TObject and TClass
  Object Pascal Language Guide

TObject and TClass

Topic groups	See also

The TObject class, declared in the System unit, is the ultimate ancestor of all other classes. TObject defines only a handful of methods, including a basic constructor and destructor. In addition to TObject, the System unit declares the class-reference type TClass:

TClass = class of TObject;

If the declaration of a class type doesn’t specify an ancestor, the class inherits directly from TObject. Thus

type TMyClass = class

 ...

end;

is equivalent to

type TMyClass = class(TObject)

 ...

end;

The latter form is recommended for readability.


Compatibility of class types
  Object Pascal Language Guide

Compatibility of class types

Topic groups	See also

A class type is assignment-compatible with its ancestors. Hence a variable of a class type can reference an instance of any descendant type. For example, given the declarations

type

  TFigure = class(TObject);

  TRectangle = class(TFigure);

  TSquare = class(TRectangle);

var

  Fig: TFigure;

the variable Fig can be assigned values of type TFigure, TRectangle, and TSquare.


Object types
  Object Pascal Language Guide

Object types

Topic groups	See also

As an alternative to class types, you can declare object types using the syntax

type objectTypeName = object (ancestorObjectType)

  memberList

end;

where objectTypeName is any valid identifier, (ancestorObjectType) is optional, and memberList declares fields, methods, and properties. If (ancestorObjectType) is omitted, then the new type has no ancestor. Object types cannot have published members.

Since object types do not descend from TObject, they provide no built-in constructors, destructors, or other methods. You can create instances of an object type using the New procedure and destroy them with the Dispose procedure, or you can simply declare variables of an object type, just as you would with records.

Object types are supported for backward compatibility only. Their use is not recommended.


Visibility of class members
  Object Pascal Language Guide

Visibility of class members

Topic groups	See also

Every member of a class has an attribute called visibility, which is indicated by one of the reserved words private, protected, public, published, or automated. For example,

published property Color: TColor read GetColor write SetColor;

declares a published property called Color. Visibility determines where and how a member can be accessed, with private representing the least accessibility, protected representing an intermediate level of accessibility, and public, published, and automated representing the greatest accessibility.

Private, protected, and public members

Published members

Automated members

If a member’s declaration appears without its own visibility specifier, the member has the same visibility as the one that precedes it. Members at the beginning of a class declaration that don’t have a specified visibility are by default published, provided the class is compiled in the {$M+} state or is derived from a class compiled in the {$M+} state; otherwise, such members are public.

For readability, it is best to organize a class declaration by visibility, placing all the private members together, followed by all the protected members, and so forth. This way each visibility reserved word appears at most once and marks the beginning of a new “section” of the declaration. So a typical class declaration should like this:

type

  TMyClass = class(TControl)

  private

   ... { private declarations here}

  protected

   ... { protected declarations here }

  public

   ... { public declarations here }

  published

   ... { published declarations here }

  end;

You can increase the visibility of a member in a descendant class by redeclaring it, but you cannot decrease its visibility. For example, a protected property can be made public in a descendant, but not private. Moreover, published members cannot become public in a descendant class. For more information, see Property overrides and redeclarations.


Private, protected, and public members
  Object Pascal Language Guide

Private, protected, and public members

Topic groups	See also

A private member is invisible outside of the unit or program where its class is declared. In other words, a private method cannot be called from another module, and a private field or property cannot be read or written to from another module. By placing related class declarations in the same module, you can give the classes access to one another’s private members without making those members more widely accessible.

A protected member is visible anywhere in the module where its class is declared and from any descendant class, regardless of the module where the descendant class appears. In other words, a protected method can be called, and a protected field or property read or written to, from the definition of any method belonging to a class that descends from the one where the protected member is declared. Members that are intended for use only in the implementation of derived classes are usually protected.

A public member is visible wherever its class can be referenced.


Published members
  Object Pascal Language Guide

Published members

Topic groups	See also

Published members have the same visibility as public members. The difference is that runtime type information (RTTI) is generated for published members. RTTI allows an application to query the fields and properties of an object dynamically and to locate its methods. RTTI is used to access the values of properties when saving and loading form files, to display properties in the Object Inspector, and to associate specific methods (called event handlers) with specific properties (called events).

Published properties are restricted to certain data types. Ordinal, string, class, interface, and method-pointer types can be published. So can set types, provided the upper and lower bounds of the base type have ordinal values between 0 and 31. (In other words, the set must fit in a byte, word, or double word.) Any real type except Real48 can be published. Properties of an array type (as distinct from array properties, discussed below) cannot be published.

Some properties, although publishable, are not fully supported by the streaming system. These include properties of record types, array properties of all publishable types, and properties of enumerated types that include anonymous values. If you publish a property of this kind, the Object Inspector won’t display it correctly, nor will the property’s value be preserved when objects are streamed to disk.

All methods are publishable, but a class cannot publish two or more overloaded methods with the same name. Fields can be published only if they are of a class or interface type.

A class cannot have published members unless it is compiled in the {$M+} state or descends from a class compiled in the {$M+} state. Most classes with published members derive from TPersistent, which is compiled in the {$M+} state, so it is seldom necessary to use the $M directive.


Automated members
  Object Pascal Language Guide

Automated members

Topic groups	See also

Automated members have the same visibility as public members. The difference is that Automation type information (required for Automation servers) is generated for automated members. Automated members typically appear only in Windows classes and is not recommended for Linux programming. The automated reserved word is maintained for backward compatibility. The TAutoObject class in the ComObj unit does not use automated.

The following restrictions apply to methods and properties declared as automated.

The types of all properties, array property parameters, method parameters, and function results must be automatable. The automatable types are Byte, Currency, Real, Double, Longint, Integer, Single, Smallint, AnsiString, WideString, TDateTime, Variant, OleVariant, WordBool, and all interface types.

Method declarations must use the default register calling convention. They can be virtual, but not dynamic.

Property declarations can include access specifiers (read and write) but other specifiers (index, stored, default, and nodefault) are not allowed. Access specifiers must list a method identifier that uses the default register calling convention; field identifiers are not allowed.

Property declarations must specify a type. Property overrides are not allowed.

The declaration of an automated method or property can include a dispid directive. Specifying an already used ID in a dispid directive causes an error.

On Windows, this directive must be followed by an integer constant that specifies an Automation dispatch ID for the member. Otherwise, the compiler automatically assigns the member a dispatch ID that is one larger than the largest dispatch ID used by any method or property in the class and its ancestors. For more information about Automation (on Windows only), see Automation objects.


Forward declarations and mutually dependent classes
  Object Pascal Language Guide

Forward declarations and mutually dependent classes

Topic groups	See also

If the declaration of a class type ends with the word class and a semicolon—that is, if it has the form

type className = class;

with no ancestor or class members listed after the word class—then it is a forward declaration. A forward declaration must be resolved by a defining declaration of the same class within the same type declaration section. In other words, between a forward declaration and its defining declaration, nothing can occur except other type declarations.

Forward declarations allow mutually dependent classes. For example,

type

  TFigure = class;  // forward declaration

  TDrawing = class

    Figure: TFigure;

     ...

  end;

  TFigure = class  // defining declaration

    Drawing: TDrawing;

     ...

  end;

Do not confuse forward declarations with complete declarations of types that derive from TObject without declaring any class members.

type

  TFirstClass = class;           // this is a forward declaration

  TSecondClass = class           // this is a complete class declaration

    end;                         //

  TThirdClass = class(TObject);  // this is a complete class declaration


Fields
  Object Pascal Language Guide

Fields

Topic groups	See also

A field is like a variable that belongs to an object. Fields can be of any type, including class types. (That is, fields can hold object references.) Fields are usually private.

To define a field member of a class, simply declare the field as you would a variable. All field declarations must occur before any property or method declarations. For example, the following declaration creates a class called TNumber whose only member, other than the methods is inherits from TObject, is an integer field called Int.

type TNumber = class

  Int: Integer;

end;

Fields are statically bound; that is, references to them are fixed at compile time. To see what this means, consider the following code.

type

  TAncestor = class

    Value: Integer;

  end;

  TDescendant = class(TAncestor)

    Value: string;  // hides the inherited Value field

  end;

var

  MyObject: TAncestor;

begin

  MyObject := TDescendant.Create;

  MyObject.Value := 'Hello!';  // error

  TDescendant(MyObject).Value := 'Hello!';  // works!

end;

Although MyObject holds an instance of TDescendant, it is declared as TAncestor. The compiler therefore interprets MyObject.Value as referring to the (integer) field declared in TAncestor. Both fields, however, exist in the TDescendant object; the inherited Value is hidden by the new one, and can be accessed through a typecast.


Methods: Overview
  Object Pascal Language Guide

Methods

Topic groups	See also

A method is a procedure or function associated with a class. A call to a method specifies the object (or, if it is a class method, the class) that the method should operate on. For example,

SomeObject.Free

calls the Free method in SomeObject.

Method declarations and implementations

Method binding

Overloading methods

Constructors

Destructors

Message handlers


Method declarations and implementations
  Object Pascal Language Guide

Method declarations and implementations

Topic groups	See also

Within a class declaration, methods appear as procedure and function headings, which work like forward declarations. Somewhere after the class declaration, but within the same module, each method must be implemented by a defining declaration. For example, suppose the declaration of TMyClass includes a method called DoSomething:

type

  TMyClass = class(TObject)

     ...

    procedure DoSomething;

     ...

  end;

A defining declaration for DoSomething must occur later in the module:

procedure TMyClass.DoSomething;

begin

 ...

end;

While a class can be declared in either the interface or the implementation section of a unit, defining declarations for a class’s methods must be in the implementation section.

In the heading of a defining declaration, the method name is always qualified with the name of the class to which it belongs. The heading can repeat the parameter list from the class declaration; if it does so, the order, type, and names of the parameters must match exactly, and, if the method is a function, so must the return value.

Method declarations can include special directives that are not used with other functions or procedures. Directives should appear in the class declaration only, not in the defining declaration, and should always be listed in the following order:

reintroduce; overload; binding; calling convention; abstract; warning

where binding is virtual, dynamic, or override; calling convention is register, pascal, cdecl, stdcall, or safecall; and warning is platform, deprecated, or library.

Inherited

Self


Inherited
  Object Pascal Language Guide

Inherited

Topic groups	See also

The reserved word inherited plays a special role in implementing polymorphic behavior. It can occur in method definitions, with or without an identifier after it.

If inherited is followed by the name of a member, it represents a normal method call or reference to a property or field—except that the search for the referenced member begins with the immediate ancestor of the enclosing method’s class. For example, when

inherited Create(...);

occurs in the definition of a method, it calls the inherited Create.

When inherited has no identifier after it, it refers to the inherited method with the same name as the enclosing method. In this case, inherited takes no explicit parameters, but passes to the inherited method the same parameters with which the enclosing method was called. For example,

inherited;

occurs frequently in the implementation of constructors. It calls the inherited constructor with the same parameters that were passed to the descendant.


Self
  Object Pascal Language Guide

Self

Topic groups	See also

Within the implementation of a method, the identifier Self references the object in which the method is called. For example, here is the implementation of TCollection’s Add method in the Classes unit.

function TCollection.Add: TCollectionItem;

begin

  Result := FItemClass.Create(Self);

end;

The Add method calls the Create method in the class referenced by the FItemClass field, which is always a TCollectionItem descendant. TCollectionItem.Create takes a single parameter of type TCollection, so Add passes it the TCollection instance object where Add is called. This is illustrated in the following code.

var MyCollection: TCollection;

 ...

MyCollection.Add  // MyCollection is passed to the TCollectionItem.Create method

Self is useful for a variety of reasons. For example, a member identifier declared in a class type might be redeclared in the block of one of the class’s methods. In this case, you can access the original member identifier as Self.Identifier.

For information about Self in class methods, see Class methods.


Method binding: Overview
  Object Pascal Language Guide

Method binding

Topic groups	See also

Methods can be static (the default), virtual, or dynamic. Virtual and dynamic methods can be overridden, and they can be abstract. These designations come into play when a variable of one class type holds a value of a descendant class type. They determine which implementation is activated when a method is called.

Static methods

Virtual and dynamic methods

Abstract methods


Static methods
  Object Pascal Language Guide

Static methods

Topic groups	See also

Methods are by default static. When a static method is called, the declared (compile-time) type of the class or object variable used in the method call determines which implementation to activate. In the following example, the Draw methods are static.

type

  TFigure = class

    procedure Draw;

  end;

  TRectangle = class(TFigure)

    procedure Draw;

  end;

Given these declarations, the following code illustrates the effect of calling a static method. In the second call to Figure.Draw, the Figure variable references an object of class TRectangle, but the call invokes the implementation of Draw in TFigure, because the declared type of the Figure variable is TFigure.

var

  Figure: TFigure;

  Rectangle: TRectangle;

begin

  Figure := TFigure.Create;

  Figure.Draw;  // calls TFigure.Draw

  Figure.Destroy;

  Figure := TRectangle.Create;

  Figure.Draw;  // calls TFigure.Draw

  TRectangle(Figure).Draw;  // calls TRectangle.Draw

  Figure.Destroy;

  Rectangle := TRectangle.Create;

  Rectangle.Draw;  // calls TRectangle.Draw

  Rectangle.Destroy;

end;


Virtual and dynamic methods
  Object Pascal Language Guide

Virtual and dynamic methods

Topic groups	See also

To make a method virtual or dynamic, include the virtual or dynamic directive in its declaration. Virtual and dynamic methods, unlike static methods, can be overridden in descendant classes. When an overridden method is called, the actual (runtime) type of the class or object used in the method call—not the declared type of the variable—determines which implementation to activate.

To override a method, redeclare it with the override directive. An override declaration must match the ancestor declaration in the order and type of its parameters and in its result type (if any).

In the following example, the Draw method declared in TFigure is overridden in two descendant classes.

type

  TFigure = class

    procedure Draw; virtual;

  end;

  TRectangle = class(TFigure)

    procedure Draw; override;

  end;

  TEllipse = class(TFigure)

    procedure Draw; override;

  end;

Given these declarations, the following code illustrates the effect of calling a virtual method through a variable whose actual type varies at runtime.

var

  Figure: TFigure;

begin

  Figure := TRectangle.Create;

  Figure.Draw;  // calls TRectangle.Draw

  Figure.Destroy;

  Figure := TEllipse.Create;

  Figure.Draw;  // calls TEllipse.Draw

  Figure.Destroy;

end;

Only virtual and dynamic methods can be overridden. All methods, however, can be overloaded; see Overloading methods.

Virtual versus dynamic

Virtual and dynamic methods are semantically equivalent. They differ only in the implementation of method-call dispatching at runtime. Virtual methods optimize for speed, while dynamic methods optimize for code size.

In general, virtual methods are the most efficient way to implement polymorphic behavior. Dynamic methods are useful when a base class declares many overridable methods which are inherited by many descendant classes in an application, but only occasionally overridden.

Overriding versus hiding

Reintroduce

Abstract methods


Overriding versus hiding
  Object Pascal Language Guide

Overriding versus hiding

Topic groups	See also

If a method declaration specifies the same method identifier and parameter signature as an inherited method, but doesn’t include override, the new declaration merely hides the inherited one without overriding it. Both methods exist in the descendant class, where the method name is statically bound. For example,

type

  T1 = class(TObject)

    procedure Act; virtual;

  end;

  T2 = class(T1)

    procedure Act;  // Act is redeclared, but not overridden

  end;

var

  SomeObject: T1;

begin

  SomeObject := T2.Create;

  SomeObject.Act;  // calls T1.Act

end;


Reintroduce
  Object Pascal Language Guide

Reintroduce

Topic groups	See also

The reintroduce directive suppresses compiler warnings about hiding previously declared virtual methods. For example,

procedure DoSomething; reintroduce;  // the ancestor class also has a DoSomething method

Use reintroduce when you want to hide an inherited virtual method with a new one.


Abstract methods
  Object Pascal Language Guide

Abstract methods

Topic groups	See also

An abstract method is a virtual or dynamic method that has no implementation in the class where it is declared. Its implementation is deferred to a descendant class. Abstract methods must be declared with the directive abstract after virtual or dynamic. For example,

procedure DoSomething; virtual; abstract;

You can call an abstract method only in a class or instance of a class in which the method has been overridden.


Overloading methods
  Object Pascal Language Guide

Overloading methods

Topic groups	See also

A method can be redeclared using the overload directive. In this case, if the redeclared method has a different parameter signature from its ancestor, it overloads the inherited method without hiding it. Calling the method in a descendant class activates whichever implementation matches the parameters in the call.

If you overload a virtual method, use the reintroduce directive when you redeclare it in descendant classes. For example,

type

  T1 = class(TObject)

    procedure Test(I: Integer); overload; virtual;

  end;

  T2 = class(T1)

    procedure Test(S: string); reintroduce; overload;

  end;

   ...

  SomeObject := T2.Create;

  SomeObject.Test('Hello!');  // calls T2.Test

  SomeObject.Test(7);         // calls T1.Test

Within a class, you cannot publish multiple overloaded methods with the same name. Maintenance of runtime type information requires a unique name for each published member.

type

  TSomeClass = class

  published

    function Func(P: Integer): Integer;

    function Func(P: Boolean): Integer  // error

     ...

Methods that serve as property read or write specifiers cannot be overloaded.

The implementation of an overloaded method must repeat the parameter list from the class declaration. For more information about overloading, see Overloading procedures and functions.


Constructors
  Object Pascal Language Guide

Constructors

Topic groups	See also

A constructor is a special method that creates and initializes instance objects. The declaration of a constructor looks like a procedure declaration, but it begins with the word constructor. Examples:

constructor Create;

constructor Create(AOwner: TComponent);

Constructors must use the default register calling convention. Although the declaration specifies no return value, a constructor returns a reference to the object it creates or is called in.

A class can have more than one constructor, but most have only one. It is conventional to call the constructor Create.

To create an object, call the constructor method in a class type. For example,

MyObject := TMyClass.Create;

This allocates storage for the new object on the heap, sets the values of all ordinal fields to zero, assigns nil to all pointer and class-type fields, and makes all string fields empty. Other actions specified in the constructor implementation are performed next; typically, objects are initialized based on values passed as parameters to the constructor. Finally, the constructor returns a reference to the newly allocated and initialized object. The type of the returned value is the same as the class type specified in the constructor call.

If an exception is raised during execution of a constructor that was invoked on a class reference, the Destroy destructor is automatically called to destroy the unfinished object.

When a constructor is called using an object reference (rather than a class reference), it does not create an object. Instead, the constructor operates on the specified object, executing only the statements in the constructor’s implementation, and then returns a reference to the object. A constructor is typically invoked on an object reference in conjunction with the reserved word inherited to execute an inherited constructor.

Here is an example of a class type and its constructor.

type

  TShape = class(TGraphicControl)

  private

    FPen: TPen;

    FBrush: TBrush;

    procedure PenChanged(Sender: TObject);

    procedure BrushChanged(Sender: TObject);

  public

    constructor Create(Owner: TComponent); override;

    destructor Destroy; override;

    ...

  end;

constructor TShape.Create(Owner: TComponent);

begin

  inherited Create(Owner);  // Initialize inherited parts

  Width := 65;  // Change inherited properties

  Height := 65;

  FPen := TPen.Create;  // Initialize new fields

  FPen.OnChange := PenChanged;

  FBrush := TBrush.Create;

  FBrush.OnChange := BrushChanged;

end;

The first action of a constructor is usually to call an inherited constructor to initialize the object’s inherited fields. The constructor then initializes the fields introduced in the descendant class. Because a constructor always clears the storage it allocates for a new object, all fields start with a value of zero (ordinal types), nil (pointer and class types), empty (string types), or Unassigned (variants). Hence there is no need to initialize fields in a constructor’s implementation except to nonzero or nonempty values.

When invoked through a class-type identifier, a constructor declared as virtual is equivalent to a static constructor. When combined with class-reference types, however, virtual constructors allow polymorphic construction of objects—that is, construction of objects whose types aren’t known at compile time. (See Class references.)


Destructors
  Object Pascal Language Guide

Destructors

Topic groups	See also

A destructor is a special method that destroys the object where it is called and deallocates its memory. The declaration of a destructor looks like a procedure declaration, but it begins with the word destructor. Examples:

destructor Destroy;

destructor Destroy; override;

Destructors must use the default register calling convention. Although a class can have more than one destructor, it is recommended that each class override the inherited Destroy method and declare no other destructors.

To call a destructor, you must reference an instance object. For example,

MyObject.Destroy;

When a destructor is called, actions specified in the destructor implementation are performed first. Typically, these consist of destroying any embedded objects and freeing resources that were allocated by the object. Then the storage that was allocated for the object is disposed of.

Here is an example of a destructor implementation.

destructor TShape.Destroy;

begin

  FBrush.Free;

  FPen.Free;

  inherited Destroy;

end;

The last action in a destructor’s implementation is typically to call the inherited destructor to destroy the object’s inherited fields.

When an exception is raised during creation of an object, Destroy is automatically called to dispose of the unfinished object. This means that Destroy must be prepared to dispose of partially constructed objects. Because a constructor sets the fields of a new object to zero or empty values before performing other actions, class-type and pointer-type fields in a partially constructed object are always nil. A destructor should therefore check for nil values before operating on class-type or pointer-type fields. Calling the Free method (defined in TObject), rather than Destroy, offers a convenient way of checking for nil values before destroying an object.


Message methods
  Object Pascal Language Guide

Message methods

Topic groups	See also

Message methods implement responses to dynamically dispatched messages. The message method syntax is supported on all platforms. The VCL uses message methods to respond to Windows messages. CLX does not use message methods to respond to system events.

A message method is created by including the message directive in a method declaration, followed by an integer constant between 1 and 49151 which specifies the message ID. For message methods in VCL controls, the integer constant can be one of the Windows message IDs defined, along with corresponding record types, in the Messages unit. A message method must be a procedure that takes a single var parameter.

For example, on Windows:

type

  TTextBox = class(TCustomControl)

  private

    procedure WMChar(var Message: TWMChar); message WM_CHAR;

    ...

  end;

For example, on Linux or for cross-platform programming, you would handle messages as follows:

const

  ID_REFRESH = $0001;

type

  TTextBox = class(TCustomControl)

  private

    procedure Refresh(var Message: TMessageRecordType); message ID_REFRESH;

    ...

  end;

A message method does not have to include the override directive to override an inherited message method. In fact, it doesn’t have to specify the same method name or parameter type as the method it overrides. The message ID alone determines which message the method responds to and whether it is an override.

Implementing message methods

The implementation of a message method can call the inherited message method, as in this example (for Windows):

procedure TTextBox.WMChar(var Message: TWMChar);

begin

  if Chr(Message.CharCode) = #13 then

    ProcessEnter

  else

    inherited;

end;

On Linux or for cross-platform programming, you would write the same example as follows:

procedure TTextBox.Refresh(var Message: TMessageRecordType);

begin

  if Chr(Message.Code) = #13 then

    ...

  else

    inherited;

end;

The inherited statement searches backward through the class hierarchy and invokes the first message method with the same ID as the current method, automatically passing the message record to it. If no ancestor class implements a message method for the given ID, inherited calls the DefaultHandler method originally defined in TObject.

The implementation of DefaultHandler in TObject simply returns without performing any actions. By overriding DefaultHandler, a class can implement its own default handling of messages. On Windows, the DefaultHandler method for VCL controls calls the Windows DefWindowProc function.

Message dispatching

Message handlers are seldom called directly. Instead, messages are dispatched to an object using the Dispatch method inherited from TObject:

procedure Dispatch(var Message);

The Message parameter passed to Dispatch must be a record whose first entry is a field of type Cardinal containing a message ID.

Dispatch searches backward through the class hierarchy (starting from the class of the object where it is called) and invokes the first message method for the ID passed to it. If no message method is found for the given ID, Dispatch calls DefaultHandler.


Properties: Overview
  Object Pascal Language Guide

Properties

Topic groups	See also

A property, like a field, defines an attribute of an object. But while a field is merely a storage location whose contents can be examined and changed, a property associates specific actions with reading or modifying its data. Properties provide control over access to an object’s attributes, and they allow attributes to be computed.

The declaration of a property specifies a name and a type, and includes at least one access specifier. The syntax of a property declaration is

property propertyName[indexes]: type index integerConstant specifiers;

where

propertyName is any valid identifier.

[indexes] is optional and is a sequence of parameter declarations separated by semicolons. Each parameter declaration has the form identifier1, ..., identifiern: type. For more information, see Array properties.

type must be a predefined or previously declared data type. That is, property declarations like property Num: 0..9 ... are invalid.

the index integerConstant clause is optional. For more information, see Index specifiers.

specifiers is a sequence of read, write, stored, default (or nodefault), and implements specifiers. Every property declaration must have at least one read or write specifier.

Properties are defined by their access specifiers. Unlike fields, properties cannot be passed as var parameters, nor can the @ operator be applied to a property. The reason is that a property doesn’t necessarily exist in memory. It could, for instance, have a read method that retrieves a value from a database or generates a random value.

Property overrides and redeclarations


Property access
  Object Pascal Language Guide

Property access

Topic groups	See also

Every property has a read specifier, a write specifier, or both. These are called access specifiers and they have the form

read fieldOrMethod

write fieldOrMethod

where fieldOrMethod is the name of a field or method declared in the same class as the property or in an ancestor class.

If fieldOrMethod is declared in the same class, it must occur before the property declaration. If it is declared in an ancestor class, it must be visible from the descendant; that is, it cannot be a private field or method of an ancestor class declared in a different unit.

If fieldOrMethod is a field, it must be of the same type as the property.

If fieldOrMethod is a method, it cannot be overloaded. Moreover, access methods for a published property must use the default register calling convention.

In a read specifier, if fieldOrMethod is a method, it must be a parameterless function whose result type is the same as the property’s type.

In a write specifier, if fieldOrMethod is a method, it must be a procedure that takes a single value or const parameter of the same type as the property.

For example, given the declaration

property Color: TColor read GetColor write SetColor;

the GetColor method must be declared as

function GetColor: TColor;

and the SetColor method must be declared as one of these:

procedure SetColor(Value: TColor);

procedure SetColor(const Value: TColor);

(The name of SetColor‘s parameter, of course, doesn’t have to be Value.)

When a property is referenced in an expression, its value is read using the field or method listed in the read specifier. When a property is referenced in an assignment statement, its value is written using the field or method listed in the write specifier.

The example below declares a class called TCompass with a published property called Heading. The value of Heading is read through the FHeading field and written through the SetHeading procedure.

type

  THeading = 0..359;

  TCompass = class(TControl)

  private

    FHeading: THeading;

    procedure SetHeading(Value: THeading);

  published

    property Heading: THeading read FHeading write SetHeading;

    ...

  end;

Given this declaration, the statements

if Compass.Heading = 180 then GoingSouth;

Compass.Heading := 135;

correspond to

if Compass.FHeading = 180 then GoingSouth;

Compass.SetHeading(135);

In the TCompass class, no action is associated with reading the Heading property; the read operation consists of retrieving the value stored in the FHeading field. On the other hand, assigning a value to the Heading property translates into a call to the SetHeading method, which, presumably, stores the new value in the FHeading field as well as performing other actions. For example, SetHeading might be implemented like this:

procedure TCompass.SetHeading(Value: THeading);

begin

  if FHeading <> Value then

  begin

    FHeading := Value;

    Repaint;  // update user interface to reflect new value

  end;

end;

A property whose declaration includes only a read specifier is a read-only property, and one whose declaration includes only a write specifier is a write-only property. It is an error to assign a value to a read-only property or use a write-only property in an expression.


Array properties
  Object Pascal Language Guide

Array properties

Topic groups	See also

Array properties are indexed properties. They can represent things like items in a list, child controls of a control, and pixels of a bitmap.

The declaration of an array property includes a parameter list that specifies the names and types of the indexes. For example,

property Objects[Index: Integer]: TObject read GetObject write SetObject;

property Pixels[X, Y: Integer]: TColor read GetPixel write SetPixel;

property Values[const Name: string]: string read GetValue write SetValue;

The format of an index parameter list is the same as that of a procedure’s or function’s parameter list, except that the parameter declarations are enclosed in brackets instead of parentheses. Unlike arrays, which can use only ordinal-type indexes, array properties allow indexes of any type.

For array properties, access specifiers must list methods rather than fields. The method in a read specifier must be a function that takes the number and type of parameters listed in the property’s index parameter list, in the same order, and whose result type is identical to the property’s type. The method in a write specifier must be a procedure that takes the number and type of parameters listed in the property’s index parameter list, in the same order, plus an additional value or const parameter of the same type as the property.

For example, the access methods for the array properties above might be declared as

function GetObject(Index: Integer): TObject;

function GetPixel(X, Y: Integer): TColor;

function GetValue(const Name: string): string;

procedure SetObject(Index: Integer; Value: TObject);

procedure SetPixel(X, Y: Integer; Value: TColor);

procedure SetValue(const Name, Value: string);

An array property is accessed by indexing the property identifier. For example, the statements

if Collection.Objects[0] = nil then Exit;

Canvas.Pixels[10, 20] := clRed;

Params.Values['PATH'] := 'C:\DELPHI\BIN';

correspond to

if Collection.GetObject(0) = nil then Exit;

Canvas.SetPixel(10, 20, clRed);

Params.SetValue('PATH', 'C:\DELPHI\BIN');

On Linux, you would use a path such as ‘/usr/local/bin’ in place of ‘C:\DELPHI\BIN’ in the above example.

The definition of an array property can be followed by the default directive, in which case the array property becomes the default property of the class. For example,

type

  TStringArray = class

  public

    property Strings[Index: Integer]: string ...; default;

    ...

  end;

If a class has a default property, you can access that property with the abbreviation object[index], which is equivalent to object.property[index]. For example, given the declaration above, StringArray.Strings[7] can be abbreviated to StringArray[7]. A class can have only one default property. Changing or hiding the default property in descendant classes may lead to unexpected behavior, since the compiler always determines an object’s default property statically.


Index specifiers
  Object Pascal Language Guide

Index specifiers

Topic groups	See also

Index specifiers allow several properties to share the same access method while representing different values. An index specifier consists of the directive index followed by an integer constant between –2147483647 and 2147483647. If a property has an index specifier, its read and write specifiers must list methods rather than fields. For example,

type

  TRectangle = class

  private

    FCoordinates: array[0..3] of Longint;

    function GetCoordinate(Index: Integer): Longint;

    procedure SetCoordinate(Index: Integer; Value: Longint);

  public

    property Left: Longint index 0 read GetCoordinate write SetCoordinate;

    property Top: Longint index 1 read GetCoordinate write SetCoordinate;

    property Right: Longint index 2 read GetCoordinate write SetCoordinate;

    property Bottom: Longint index 3 read GetCoordinate write SetCoordinate;

    property Coordinates[Index: Integer]: Longint read GetCoordinate write SetCoordinate;

    ...

  end;

An access method for a property with an index specifier must take an extra value parameter of type Integer. For a read function, it must be the last parameter; for a write procedure, it must be the second-to-last parameter (preceding the parameter that specifies the property value). When a program accesses the property, the property’s integer constant is automatically passed to the access method.

Given the declaration above, if Rectangle is of type TRectangle, then

Rectangle.Right := Rectangle.Left + 100;

corresponds to

Rectangle.SetCoordinate(2, Rectangle.GetCoordinate(0) + 100);


Storage specifiers
  Object Pascal Language Guide

Storage specifiers

Topic groups	See also

The optional stored, default, and nodefault directives are called storage specifiers. They have no effect on program behavior, but control the way runtime type information (RTTI) is maintained. Specifically, storage specifiers determine whether or not to save the values of published properties in form files.

The stored directive must be followed by True, False, the name of a Boolean field, or the name of a parameterless method that returns a Boolean value. For example,

property Name: TComponentName read FName write SetName stored False;

If a property has no stored directive, it is treated as if stored True were specified.

The default directive must be followed by a constant of the same type as the property. For example,

property Tag: Longint read FTag write FTag default 0;

To override an inherited default value without specifying a new one, use the nodefault directive. The default and nodefault directives are supported only for ordinal types and for set types, provided the upper and lower bounds of the set’s base type have ordinal values between 0 and 31; if such a property is declared without default or nodefault, it is treated as if nodefault were specified. For reals, pointers, and strings, there is an implicit default value of 0, nil, and '' (the empty string), respectively.

When saving a component’s state, the storage specifiers of the component’s published properties are checked. If a property’s current value is different from its default value (or if there is no default value) and the stored specifier is True, then the property’s value is saved. Otherwise, the property’s value is not saved.

Note:	Storage specifiers are not supported for array properties. The default directive has a different meaning when used in an array property declaration. See Array properties.


Property overrides and redeclarations
  Object Pascal Language Guide

Property overrides and redeclarations

Topic groups	See also

A property declaration that doesn’t specify a type is called a property override. Property overrides allow you to change a property’s inherited visibility or specifiers. The simplest override consists only of the reserved word property followed by an inherited property identifier; this form is used to change a property’s visibility. For example, if an ancestor class declares a property as protected, a derived class can redeclare it in a public or published section of the class. Property overrides can include read, write, stored, default, and nodefault directives; any such directive overrides the corresponding inherited directive. An override can replace an inherited access specifier, add a missing specifier, or increase a property’s visibility, but it cannot remove an access specifier or decrease a property’s visibility. An override can include an implements directive, which adds to the list of implemented interfaces without removing inherited ones.

The following declarations illustrate the use of property overrides.

type

  TAncestor = class

   ...

  protected

    property Size: Integer read FSize;

    property Text: string read GetText write SetText;

    property Color: TColor read FColor write SetColor stored False;

    ...

  end;

type

  TDerived = class(TAncestor)

   ...

  protected

    property Size write SetSize;

  published

    property Text;

    property Color stored True default clBlue;

    ...

  end;

The override of Size adds a write specifier to allow the property to be modified. The overrides of Text and Color change the visibility of the properties from protected to published. The property override of Color also specifies that the property should be filed if its value isn’t clBlue.

A redeclaration of a property that includes a type identifier hides the inherited property rather than overriding it. This means that a new property is created with the same name as the inherited one. Any property declaration that specifies a type must be a complete declaration, and must therefore include at least one access specifier.

Whether a property is hidden or overridden in a derived class, property look-up is always static. That is, the declared (compile-time) type of the variable used to identify an object determines the interpretation of its property identifiers. Hence, after the following code executes, reading or assigning a value to MyObject.Value invokes Method1 or Method2, even though MyObject holds an instance of TDescendant. But you can cast MyObject to TDescendant to access the descendant class’s properties and their access specifiers.

type

  TAncestor = class

     ...

    property Value: Integer read Method1 write Method2;

  end;

  TDescendant = class(TAncestor)

     ...

    property Value: Integer read Method3 write Method4;

  end;

var MyObject: TAncestor;

 ...

MyObject := TDescendant.Create;


Class references: Overview
  Object Pascal Language Guide

Class references

Topic groups	See also

Sometimes operations are performed on a class itself, rather than on instances of a class (that is, objects). This happens, for example, when you call a constructor method using a class reference. You can always refer to a specific class using its name, but at times it is necessary to declare variables or parameters that take classes as values, and in these situations you need class-reference types.

Class-reference types

Class operators

Class methods


Class-reference types
  Object Pascal Language Guide

Class-reference types

Topic groups	See also

A class-reference type, sometimes called a metaclass, is denoted by a construction of the form

class of type

where type is any class type. The identifier type itself denotes a value whose type is class of type. If type1 is an ancestor of type2, then class of type2 is assignment-compatible with class of type1. Thus

type TClass = class of TObject;

var AnyObj: TClass;

declares a variable called AnyObj that can hold a reference to any class. (The definition of a class-reference type cannot occur directly in a variable declaration or parameter list.) You can assign the value nil to a variable of any class-reference type.

To see how class-reference types are used, look at the declaration of the constructor for TCollection (in the Classes unit):

type TCollectionItemClass = class of TCollectionItem;

  ...

constructor Create(ItemClass: TCollectionItemClass);

This declaration says that to create a TCollection instance object, you must pass to the constructor the name of a class descending from TCollectionItem.

Class-reference types are useful when you want to invoke a class method or virtual constructor on a class or object whose actual type is unknown at compile time.

Constructors and class references


Constructors and class references
  Object Pascal Language Guide

Constructors and class references

Topic groups	See also

A constructor can be called using a variable of a class-reference type. This allows construction of objects whose type isn’t known at compile time. For example,

type TControlClass = class of TControl;

function CreateControl(ControlClass: TControlClass;

  const ControlName: string; X, Y, W, H: Integer): TControl;

begin

  Result := ControlClass.Create(MainForm);

  with Result do

  begin

    Parent := MainForm;

    Name := ControlName;

    SetBounds(X, Y, W, H);

    Visible := True;

  end;

end;

The CreateControl function requires a class-reference parameter to tell it what kind of control to create. It uses this parameter to call the class’s constructor. Because class-type identifiers denote class-reference values, a call to CreateControl can specify the identifier of the class to create an instance of. For example,

CreateControl(TEdit, 'Edit1', 10, 10, 100, 20);

Constructors called using class references are usually virtual. The constructor implementation activated by the call depends on the runtime type of the class reference.


Class operators: Overview
  Object Pascal Language Guide

Class operators

Topic groups	See also

Every class inherits from TObject methods called ClassType and ClassParent that return, respectively, a reference to the class of an object and of an object’s immediate ancestor. Both methods return a value of type TClass (where TClass = class of TObject), which can be cast to a more specific type. Every class also inherits a method called InheritsFrom that tests whether the object where it is called descends from a specified class. These methods are used by the is and as operators, and it is seldom necessary to call them directly.

The is operator

The as operator


The is operator
  Object Pascal Language Guide

The is operator

Topic groups	See also

The is operator, which performs dynamic type checking, is used to verify the actual runtime class of an object. The expression

object is class

returns True if object is an instance of the class denoted by class or one of its descendants, and False otherwise. (If object is nil, the result is False.) If the declared type of object is unrelated to class—that is, if the types are distinct and one is not an ancestor of the other—a compilation error results. For example,

if ActiveControl is TEdit then TEdit(ActiveControl).SelectAll;

This statement casts a variable to TEdit after first verifying that the object it references is an instance of TEdit or one of its descendants.


The as operator
  Object Pascal Language Guide

The as operator

Topic groups	See also

The as operator performs checked typecasts. The expression

object as class

returns a reference to the same object as object, but with the type given by class. At runtime, object must be an instance of the class denoted by class or one of its descendants, or be nil; otherwise an exception is raised. If the declared type of object is unrelated to class—that is, if the types are distinct and one is not an ancestor of the other—a compilation error results. For example,

with Sender as TButton do

begin

  Caption := '&Ok';

  OnClick := OkClick;

end;

The rules of operator precedence often require as typecasts to be enclosed in parentheses. For example,

(Sender as TButton).Caption := '&Ok';


Class methods
  Object Pascal Language Guide

Class methods

Topic groups	See also

A class method is a method (other than a constructor) that operates on classes instead of objects. The definition of a class method must begin with the reserved word class. For example,

type

  TFigure = class

  public

    class function Supports(Operation: string): Boolean; virtual;

    class procedure GetInfo(var Info: TFigureInfo); virtual;

    ...

  end;

The defining declaration of a class method must also begin with class. For example,

class procedure TFigure.GetInfo(var Info: TFigureInfo);

begin

  ...

end;

In the defining declaration of a class method, the identifier Self represents the class where the method is called (which could be a descendant of the class in which it is defined). If the method is called in the class C, then Self is of the type class of C. Thus you cannot use Self to access fields, properties, and normal (object) methods, but you can use it to call constructors and other class methods.

A class method can be called through a class reference or an object reference. When it is called through an object reference, the class of the object becomes the value of Self.


Exceptions: Overview
  Object Pascal Language Guide

Exceptions

Topic groups	See also

An exception is raised when an error or other event interrupts normal execution of a program. The exception transfers control to an exception handler, which allows you to separate normal program logic from error-handling. Because exceptions are objects, they can be grouped into hierarchies using inheritance, and new exceptions can be introduced without affecting existing code. An exception can carry information, such as an error message, from the point where it is raised to the point where it is handled.

When an application uses the SysUtils unit, all runtime errors are automatically converted into exceptions. Errors that would otherwise terminate an application—such as insufficient memory, division by zero, and general protection faults—can be caught and handled.

When to use exceptions

Declaring exception types

Raising and handling exceptions

Standard exception classes and routines


When to use exceptions
  Object Pascal Language Guide

When to use exceptions

Topic groups	See also

Exceptions provide an elegant way to trap runtime errors without halting the program and without awkward conditional statements. The complexity of Object Pascal’s exception-handling mechanism, however, makes it inefficient, and it should therefore be used judiciously. While it is possible to raise exceptions for almost any reason, and to protect almost any block of code by wrapping it in a try...except or try...finally statement, in practice these tools are best reserved for special situations.

Exception handling is appropriate for errors whose chances of occurring are low or difficult to assess, but whose consequences are likely to be catastrophic (such as crashing the application); for error conditions that are complicated or difficult to test for in if...then statements; and when you need to respond to exceptions raised by the operating system or by routines whose source code you don’t control. Exceptions are commonly used for hardware, memory, I/O, and operating-system errors.

Conditional statements are often the best way to test for errors. For example, suppose you want to make sure that a file exists before trying to open it. You could do it this way:

try

  AssignFile(F, FileName);

  Reset(F);  // raises an EInOutError exception if file is not found

except

  on Exception do ...

end;

But you could also avoid the overhead of exception handling by using

if FileExists(FileName) then  // returns False if file is not found; raises no exception

begin

  AssignFile(F, FileName);

  Reset(F);

end;

Assertions provide another way of testing a Boolean condition anywhere in your source code. When an Assert statement fails, the program either halts or (if it uses the SysUtils unit) raises an EAssertionFailed exception. Assertions should be used only to test for conditions that you do not expect to occur. For more information, see the online Help for the standard procedure Assert.


Declaring exception types
  Object Pascal Language Guide

Declaring exception types

Topic groups	See also

Exception types are declared just like other classes. In fact, it is possible to use an instance of any class as an exception, but it is recommended that exceptions be derived from the Exception class defined in SysUtils.

You can group exceptions into families using inheritance. For example, the following declarations in SysUtils define a family of exception types for math errors.

type

  EMathError = class(Exception);

  EInvalidOp = class(EMathError);

  EZeroDivide = class(EMathError);

  EOverflow = class(EMathError);

  EUnderflow = class(EMathError);

Given these declarations, you can define a single EMathError exception handler that also handles EInvalidOp, EZeroDivide, EOverflow, and EUnderflow.

Exception classes sometimes define fields, methods, or properties that convey additional information about the error. For example,

type EInOutError = class(Exception)

    ErrorCode: Integer;

  end;


Raising and handling exceptions
  Object Pascal Language Guide

Raising and handling exceptions

Topic groups	See also

To create an exception object, call the exception class’s constructor within a raise statement. For example,

raise EMathError.Create;

In general, the form of a raise statement is

raise object at address

where object and at address are both optional. If object is omitted, the statement re-raises the current exception; see Re-raising exceptions. When an address is specified, it is usually a pointer to a procedure or function; use this option to raise the exception from an earlier point in the stack than the one where the error actually occurred.

When an exception is raised—that is, referenced in a raise statement—it is governed by special exception-handling logic. A raise statement never returns control in the normal way. Instead, it transfers control to the innermost exception handler that can handle exceptions of the given class. (The innermost handler is the one whose try...except block was most recently entered but has not yet exited.)

For example, the function below converts a string to an integer, raising an ERangeError exception if the resulting value is outside a specified range.

function StrToIntRange(const S: string; Min, Max: Longint): Longint;

begin

  Result := StrToInt(S);  // StrToInt is declared in SysUtils

  if (Result < Min) or (Result > Max) then

    raise ERangeError.CreateFmt(

      '%d is not within the valid range of %d..%d',

      [Result, Min, Max]);

end;

Notice the CreateFmt method called in the raise statement. Exception and its descendants have special constructors that provide alternative ways to create exception messages and context IDs.

A raised exception is destroyed automatically after it is handled. Never attempt to destroy a raised exception manually.

Note:	Raising an exception in the initialization section of a unit may not produce the intended result. Normal exception support comes from the SysUtils unit, which must be initialized before such support is available. If an exception occurs during initialization, all initialized units—including SysUtils—are finalized and the exception is re-raised. Then the exception is caught and handled, usually by interrupting the program.

Try...except statements

Nested exceptions

Try...finally statements


Try...except statements
  Object Pascal Language Guide

Try...except statements

Topic groups	See also

Exceptions are handled within try...except statements. For example,

try

  X := Y/Z;

except

  on EZeroDivide do HandleZeroDivide;

end;

This statement attempts to divide Y by Z, but calls a routine named HandleZeroDivide if an EZeroDivide exception is raised.

The syntax of a try...except statement is

try statements except exceptionBlock end

where statements is a sequence of statements (delimited by semicolons) and exceptionBlock is either

another sequence of statements or

a sequence of exception handlers, optionally followed by

else statements

An exception handler has the form

on identifier: type do statement

where identifier: is optional (if included, identifier can be any valid identifier), type is a type used to represent exceptions, and statement is any statement.

A try...except statement executes the statements in the initial statements list. If no exceptions are raised, the exception block (exceptionBlock) is ignored and control passes to the next part of the program.

If an exception is raised during execution of the initial statements list, either by a raise statement in the statements list or by a procedure or function called from the statements list, an attempt is made to “handle” the exception:

If any of the handlers in the exception block matches the exception, control passes to the first such handler. An exception handler “matches” an exception just in case the type in the handler is the class of the exception or an ancestor of that class.

If no such handler is found, control passes to the statement in the else clause, if there is one.

If the exception block is just a sequence of statements without any exception handlers, control passes to the first statement in the list.

If none of the conditions above is satisfied, the search continues in the exception block of the next-most-recently entered try...except statement that has not yet exited. If no appropriate handler, else clause, or statement list is found there, the search propagates to the next-most-recently entered try...except statement, and so forth. If the outermost try...except statement is reached and the exception is still not handled, the program terminates.

When an exception is handled, the stack is traced back to the procedure or function containing the try...except statement where the handling occurs, and control is transferred to the executed exception handler, else clause, or statement list. This process discards all procedure and function calls that occurred after entering the try...except statement where the exception is handled. The exception object is then automatically destroyed through a call to its Destroy destructor and control is passed to the statement following the try...except statement. (If a call to the Exit, Break, or Continue standard procedure causes control to leave the exception handler, the exception object is still automatically destroyed.)

In the example below, the first exception handler handles division-by-zero exceptions, the second one handles overflow exceptions, and the final one handles all other math exceptions. EMathError appears last in the exception block because it is the ancestor of the other two exception classes; if it appeared first, the other two handlers would never be invoked.

try

 ...

except

  on EZeroDivide do HandleZeroDivide;

  on EOverflow do HandleOverflow;

  on EMathError do HandleMathError;

end;

An exception handler can specify an identifier before the name of the exception class. This declares the identifier to represent the exception object during execution of the statement that follows on...do. The scope of the identifier is limited to that statement. For example,

try

 ...

except

  on E: Exception do ErrorDialog(E.Message, E.HelpContext);

end;

If the exception block specifies an else clause, the else clause handles any exceptions that aren’t handled by the block’s exception handlers. For example,

try

 ...

except

  on EZeroDivide do HandleZeroDivide;

  on EOverflow do HandleOverflow;

  on EMathError do HandleMathError;

else

  HandleAllOthers;

end;

Here, the else clause handles any exception that isn’t an EMathError.

An exception block that contains no exception handlers, but instead consists only of a list of statements, handles all exceptions. For example,

try

 ...

except

  HandleException;

end;

Here, the HandleException routine handles any exception that occurs as a result of executing the statements between try and except.


Re-raising exceptions
  Object Pascal Language Guide

Re-raising exceptions

Topic groups	See also

When the reserved word raise occurs in an exception block without an object reference following it, it raises whatever exception is handled by the block. This allows an exception handler to respond to an error in a limited way and then re-raise the exception. Re-raising is useful when a procedure or function has to clean up after an exception occurs but cannot fully handle the exception.

For example, the GetFileList function allocates a TStringList object and fills it with file names matching a specified search path:

function GetFileList(const Path: string): TStringList;

var

  I: Integer;

  SearchRec: TSearchRec;

begin

  Result := TStringList.Create;

  try

    I := FindFirst(Path, 0, SearchRec);

    while I = 0 do

    begin

      Result.Add(SearchRec.Name);

      I := FindNext(SearchRec);

    end;

  except

    Result.Free;

    raise;

  end;

end;

GetFileList creates a TStringList object, then uses the FindFirst and FindNext functions (defined in SysUtils) to initialize it. If the initialization fails—for example because the search path is invalid, or because there is not enough memory to fill in the string list—GetFileList needs to dispose of the new string list, since the caller does not yet know of its existence. For this reason, initialization of the string list is performed in a try...except statement. If an exception occurs, the statement’s exception block disposes of the string list, then re-raises the exception.


Nested exceptions
  Object Pascal Language Guide

Nested exceptions

Topic groups	See also

Code executed in an exception handler can itself raise and handle exceptions. As long as these exceptions are also handled within the exception handler, they do not affect the original exception. However, once an exception raised in an exception handler propagates beyond that handler, the original exception is lost. This is illustrated by the Tan function below.

type

  ETrigError = class(EMathError);

function Tan(X: Extended): Extended;

begin

  try

    Result := Sin(X) / Cos(X);

  except

    on EMathError do

      raise ETrigError.Create('Invalid argument to Tan');

  end;

end;

If an EMathError exception occurs during execution of Tan, the exception handler raises an ETrigError. Since Tan does not provide a handler for ETrigError, the exception propagates beyond the original exception handler, causing the EMathError exception to be destroyed. To the caller, it appears as if the Tan function has raised an ETrigError exception.


Try...finally statements
  Object Pascal Language Guide

Try...finally statements

Topic groups	See also

Sometimes you want to ensure that specific parts of an operation are completed, whether or not the operation is interrupted by an exception. For example, when a routine acquires control of a resource, it is often important that the resource be released, regardless of whether the routine terminates normally. In these situations, you can use a try...finally statement.

The following example shows how code that opens and processes a file can ensure that the file is ultimately closed, even if an error occurs during execution.

Reset(F);

try

 ...  // process file F

finally

  CloseFile(F);

end;

The syntax of a try...finally statement is

try statementList1 finally statementList2 end

where each statementList is a sequence of statements delimited by semicolons. The try...finally statement executes the statements in statementList1 (the try clause). If statementList1 finishes without raising exceptions, statementList2 (the finally clause) is executed. If an exception is raised during execution of statementList1, control is transferred to statementList2; once statementList2 finishes executing, the exception is re-raised. If a call to the Exit, Break, or Continue procedure causes control to leave statementList1, statementList2 is automatically executed. Thus the finally clause is always executed, regardless of how the try clause terminates.

If an exception is raised but not handled in the finally clause, that exception is propagated out of the try...finally statement, and any exception already raised in the try clause is lost. The finally clause should therefore handle all locally raised exceptions, so as not to disturb propagation of other exceptions.


Standard exception classes and routines
  Object Pascal Language Guide

Standard exception classes and routines

Topic groups	See also

The SysUtils unit declares several standard routines for handling exceptions, including ExceptObject, ExceptAddr, and ShowException. SysUtils and other units also include dozens of exception classes, all of which (aside from OutlineError) derive from Exception.

The Exception class has properties called Message and HelpContext that can be used to pass an error description and a context ID for context-sensitive online documentation. It also defines various constructor methods that allow you to specify the description and context ID in different ways.


Standard routines and I/O: Overview
  Object Pascal Language Guide

Standard routines and I/O

Topic groups	See also

These topics discuss text and file I/O and summarize standard library routines. Many of the procedures and functions listed here are defined in the System unit, which is implicitly compiled with every application. Others are built into the compiler but are treated as if they were in the System unit.

Some standard routines are in units such as SysUtils, which must be listed in a uses clause to make them available in programs. You cannot, however, list System in a uses clause, nor should you modify the System unit or try to rebuild it explicitly.

File input and output

Text-file device drivers

Handling null-terminated strings

Other standard routines


File input and output
  Object Pascal Language Guide

File input and output

Topic groups	See also

The table below lists input and output routines.

Procedure or function	Description
Append	Opens an existing text file for appending.
AssignFile	Assigns the name of an external file to a file variable.
BlockRead	Reads one or more records from an untyped file.
BlockWrite	Writes one or more records into an untyped file.
ChDir	Changes the current directory.
CloseFile	Closes an open file.
Eof	Returns the end-of-file status of a file.
Eoln	Returns the end-of-line status of a text file.
Erase	Erases an external file.
FilePos	Returns the current file position of a typed or untyped file.
FileSize	Returns the current size of a file; not used for text files.
Flush	Flushes the buffer of an output text file.
GetDir	Returns the current directory of a specified drive.
IOResult	Returns an integer value that is the status of the last I/O function performed.
MkDir	Creates a subdirectory.
Read	Reads one or more values from a file into one or more variables.
Readln	Does what Read does and then skips to beginning of next line in the text file.
Rename	Renames an external file.
Reset	Opens an existing file.
Rewrite	Creates and opens a new file.
RmDir	Removes an empty subdirectory.
Seek	Moves the current position of a typed or untyped file to a specified component. Not used with text files.
SeekEof	Returns the end-of-file status of a text file.
SeekEoln	Returns the end-of-line status of a text file.
SetTextBuf	Assigns an I/O buffer to a text file.
Truncate	Truncates a typed or untyped file at the current file position.
Write	Writes one or more values to a file.
Writeln	Does the same as Write, and then writes an end-of-line marker to the text file.
A file variable is any variable whose type is a file type. There are three classes of file: typed, text, and untyped. The syntax for declaring file types is given in File types.

Before a file variable can be used, it must be associated with an external file through a call to the AssignFile procedure. An external file is typically a named disk file, but it can also be a device, such as the keyboard or the display. The external file stores the information written to the file or supplies the information read from the file.

Once the association with an external file is established, the file variable must be “opened” to prepare it for input or output. An existing file can be opened via the Reset procedure, and a new file can be created and opened via the Rewrite procedure. Text files opened with Reset are read-only and text files opened with Rewrite and Append are write-only. Typed files and untyped files always allow both reading and writing regardless of whether they were opened with Reset or Rewrite.

Every file is a linear sequence of components, each of which has the component type (or record type) of the file. The components are numbered starting with zero.

Files are normally accessed sequentially. That is, when a component is read using the standard procedure Read or written using the standard procedure Write, the current file position moves to the next numerically ordered file component. Typed files and untyped files can also be accessed randomly through the standard procedure Seek, which moves the current file position to a specified component. The standard functions FilePos and FileSize can be used to determine the current file position and the current file size.

When a program completes processing a file, the file must be closed using the standard procedure CloseFile. After a file is closed, its associated external file is updated. The file variable can then be associated with another external file.

By default, all calls to standard I/O procedures and functions are automatically checked for errors, and if an error occurs an exception is raised (or the program is terminated if exception handling is not enabled). This automatic checking can be turned on and off using the {$I+} and {$I–} compiler directives. When I/O checking is off—that is, when a procedure or function call is compiled in the {$I–} state—an I/O error doesn’t cause an exception to be raised; to check the result of an I/O operation, you must call the standard function IOResult instead.

You must call the IOResult function to clear an error, even if you aren’t interested in the error. If you don’t clear an error and {$I+} is the current state, the next I/O function call will fail with the lingering IOResult error.

Text files

Untyped files


Text files
  Object Pascal Language Guide

Text files

Topic groups	See also

This section summarizes I/O using file variables of the standard type Text.

When a text file is opened, the external file is interpreted in a special way: It is considered to represent a sequence of characters formatted into lines, where each line is terminated by an end-of-line marker (a carriage-return character, possibly followed by a linefeed character). The type Text is distinct from the type file of Char.

For text files, there are special forms of Read and Write that let you read and write values that are not of type Char. Such values are automatically translated to and from their character representation. For example, Read(F, I), where I is a type Integer variable, reads a sequence of digits, interprets that sequence as a decimal integer, and stores it in I.

There are two standard text-file variables, Input and Output. The standard file variable Input is a read-only file associated with the operating system’s standard input (typically, the keyboard). The standard file variable Output is a write-only file associated with the operating system’s standard output (typically, the display). Before an application begins executing, Input and Output are automatically opened, as if the following statements were executed:

AssignFile(Input, '');

Reset(Input);

AssignFile(Output, '');

Rewrite(Output);

Note:	Text-oriented I/O is available only in console applications—that is, applications compiled with the “Generate console application” option checked on the Linker page of the Project Options dialog box or with the -cc command-line compiler option. In a GUI (non-console) application, any attempt to read or write using Input or Output will produce an I/O error.

Some of the standard I/O routines that work on text files don’t need to have a file variable explicitly given as a parameter. If the file parameter is omitted, Input or Output is assumed by default, depending on whether the procedure or function is input- or output-oriented. For example, Read(X) corresponds to Read(Input, X) and Write(X) corresponds to Write(Output, X).

If you do specify a file when calling one of the input or output routines that work on text files, the file must be associated with an external file using AssignFile, and opened using Reset, Rewrite, or Append. An exception is raised if you pass a file that was opened with Reset to an output-oriented procedure or function. An exception is also raised if you pass a file that was opened with Rewrite or Append to an input-oriented procedure or function.


Untyped files
  Object Pascal Language Guide

Untyped files

Topic groups	See also

Untyped files are low-level I/O channels used primarily for direct access to disk files regardless of type and structuring. An untyped file is declared with the word file and nothing more. For example,

var DataFile: file;

For untyped files, the Reset and Rewrite procedures allow an extra parameter to specify the record size used in data transfers. For historical reasons, the default record size is 128 bytes. A record size of 1 is the only value that correctly reflects the exact size of any file. (No partial records are possible when the record size is 1.)

Except for Read and Write, all typed-file standard procedures and functions are also allowed on untyped files. Instead of Read and Write, two procedures called BlockRead and BlockWrite are used for high-speed data transfers.


Text-file device drivers: Overview
  Object Pascal Language Guide

Text-file device drivers

Topic groups	See also

You can define your own text-file device drivers for your programs. A text-file device driver is a set of four functions that completely implement an interface between Object Pascal’s file system and some device.

The four functions that define each device driver are Open, InOut, Flush, and Close. The function header of each function is

function DeviceFunc(var F: TTextRec): Integer;

where DeviceFunc is the name of the function (that is, Open, InOut, Flush, or Close). The return value of a device-interface function becomes the value returned by IOResult. If the return value is zero, the operation was successful.

To associate the device-interface functions with a specific file, you must write a customized Assign procedure. The Assign procedure must assign the addresses of the four device-interface functions to the four function pointers in the text-file variable. In addition, it should store the fmClosed “magic” constant in the Mode field, store the size of the text-file buffer in BufSize, store a pointer to the text-file buffer in BufPtr, and clear the Name string.

Assuming, for example, that the four device-interface functions are called DevOpen, DevInOut, DevFlush, and DevClose, the Assign procedure might look like this:

procedure AssignDev(var F: Text);

begin

  with TTextRec(F) do

  begin

    Mode := fmClosed;

    BufSize := SizeOf(Buffer);

    BufPtr := @Buffer;

    OpenFunc := @DevOpen;

    InOutFunc := @DevInOut;

    FlushFunc := @DevFlush;

    CloseFunc := @DevClose;

    Name[0] := #0;

  end;

end;

The device-interface functions can use the UserData field in the file record to store private information. This field isn’t modified by the product file system at any time.

Device functions


Device functions
  Object Pascal Language Guide

Device functions

Topic groups	See also

The functions that make up a text-file device driver are described below.

The Open function

The Open function is called by the Reset, Rewrite, and Append standard procedures to open a text file associated with a device. On entry, the Mode field contains fmInput, fmOutput, or fmInOut to indicate whether the Open function was called from Reset, Rewrite, or Append.

The Open function prepares the file for input or output, according to the Mode value. If Mode specified fmInOut (indicating that Open was called from Append), it must be changed to fmOutput before Open returns.

Open is always called before any of the other device-interface functions. For that reason, AssignDev only initializes the OpenFunc field, leaving initialization of the remaining vectors up to Open. Based on Mode, Open can then install pointers to either input- or output-oriented functions. This saves the InOut, Flush functions and the CloseFile procedure from determining the current mode.

The InOut function

The InOut function is called by the Read, Readln, Write, Writeln, Eof, Eoln, SeekEof, SeekEoln, and CloseFile standard routines whenever input or output from the device is required.

When Mode is fmInput, the InOut function reads up to BufSize characters into BufPtr^, and returns the number of characters read in BufEnd. In addition, it stores zero in BufPos. If the InOut function returns zero in BufEnd as a result of an input request, Eof becomes True for the file.

When Mode is fmOutput, the InOut function writes BufPos characters from BufPtr^, and returns zero in BufPos.

The Flush function

The Flush function is called at the end of each Read, Readln, Write, and Writeln. It can optionally flush the text-file buffer.

If Mode is fmInput, the Flush function can store zero in BufPos and BufEnd to flush the remaining (unread) characters in the buffer. This feature is seldom used.

If Mode is fmOutput, the Flush function can write the contents of the buffer exactly like the InOut function, which ensures that text written to the device appears on the device immediately. If Flush does nothing, the text doesn’t appear on the device until the buffer becomes full or the file is closed.

The Close function

The Close function is called by the CloseFile standard procedure to close a text file associated with a device. (The Reset, Rewrite, and Append procedures also call Close if the file they are opening is already open.) If Mode is fmOutput, then before calling Close, the file system calls the InOut function to ensure that all characters have been written to the device.


Handling null-terminated strings
  Object Pascal Language Guide

Handling null-terminated strings

Topic groups	See also

Object Pascal’s extended syntax allows the Read, Readln, Str, and Val standard procedures to be applied to zero-based character arrays, and allows the Write, Writeln, Val, AssignFile, and Rename standard procedures to be applied to both zero-based character arrays and character pointers. In addition, the following functions are provided for handling null-terminated strings. For more information about null-terminated strings, see Working with null-terminated strings.

Function	Description
StrAlloc	Allocates a character buffer of a given size on the heap.
StrBufSize	Returns the size of a character buffer allocated using StrAlloc or StrNew.
StrCat	Concatenates two strings.
StrComp	Compares two strings.
StrCopy	Copies a string.
StrDispose	Disposes a character buffer allocated using StrAlloc or StrNew.
StrECopy	Copies a string and returns a pointer to the end of the string.
StrEnd	Returns a pointer to the end of a string.
StrFmt	Formats one or more values into a string.
StrIComp	Compares two strings without case sensitivity.
StrLCat	Concatenates two strings with a given maximum length of the resulting string.
StrLComp	Compares two strings for a given maximum length.
StrLCopy	Copies a string up to a given maximum length.
StrLen	Returns the length of a string.
StrLFmt	Formats one or more values into a string with a given maximum length.
StrLIComp	Compares two strings for a given maximum length without case sensitivity.
StrLower	Converts a string to lowercase.
StrMove	Moves a block of characters from one string to another.
StrNew	Allocates a string on the heap.
StrPCopy	Copies a Pascal string to a null-terminated string.
StrPLCopy	Copies a Pascal string to a null-terminated string with a given maximum length.
StrPos	Returns a pointer to the first occurrence of a given substring within a string.
StrRScan	Returns a pointer to the last occurrence of a given character within a string.
StrScan	Returns a pointer to the first occurrence of a given character within a string.
StrUpper	Converts a string to uppercase.
Standard string-handling functions have multibyte-enabled counterparts that also implement locale-specific ordering for characters. Names of multibyte functions start with Ansi-. For example, the multibyte version of StrPos is AnsiStrPos. Multibyte character support is operating-system dependent and based on the current locale.

Wide-character strings

The System unit provides three functions, WideCharToString, WideCharLenToString, and StringToWideChar, that can be used to convert null-terminated wide character strings to single- or double-byte long strings.

For more information about wide-character strings, see About extended character sets.


Other standard routines
  Object Pascal Language Guide

Other standard routines

Topic groups	See also

The table below lists frequently used procedures and functions found in Borland product libraries. This is not an exhaustive inventory of standard routines.

Procedure or function	Description
Abort	Ends the process without reporting an error.
Addr	Returns a pointer to a specified object.
AllocMem	Allocates a memory block and initializes each byte to zero.
ArcTan	Calculates the arctangent of the given number.
Assert	Tests whether a boolean expression is True.
Assigned	Tests for a nil (unassigned) pointer or procedural variable.
Beep	Generates a standard beep using the computer speaker.
Break	Causes control to exit a for, while, or repeat statement.
ByteToCharIndex	Returns the position of the character containing a specified byte in a string.
Chr	Returns the character for a specified value.
Close	Terminates the association between a file variable and an external file.
CompareMem	Performs a binary comparison of two memory images.
CompareStr	Compares strings case sensitively.
CompareText	Compares strings by ordinal value and is not case sensitive.
Continue	Returns control to the next iteration of for, while, or repeat statements.
Copy	Returns a substring of a string or a segment of a dynamic array.
Cos	Calculates the cosine of an angle.
CurrToStr	Converts a currency variable to a string.
Date	Returns the current date.
DateTimeToStr	Converts a variable of type TDateTime to a string.
DateToStr	Converts a variable of type TDateTime to a string.
Dec	Decrements an ordinal variable.
Dispose	Releases memory allocated for a dynamic variable.
ExceptAddr	Returns the address at which the current exception was raised.
Exit	Exits from the current procedure.
Exp	Calculates the exponential of X.
FillChar	Fills contiguous bytes with a specified value.
Finalize	Uninitializes a dynamically allocated variable.
FloatToStr	Converts a floating point value to a string.
FloatToStrF	Converts a floating point value to a string, using specified format.
FmtLoadStr	Returns formatted output using a resourced format string.
FmtStr	Assembles a formatted string from a series of arrays.
Format	Assembles a string from a format string and a series of arrays.
FormatDateTime	Formats a date-and-time value.
FormatFloat	Formats a floating point value.
FreeMem	Disposes of a dynamic variable.
GetMem	Creates a dynamic variable and a pointer to the address of the block.
GetParentForm	Returns the form or property page that contains a specified control.
Halt	Initiates abnormal termination of a program.
Hi	Returns the high-order byte of an expression as an unsigned value.
High	Returns the highest value in the range of a type, array, or string.
Inc	Increments an ordinal variable.
Initialize	Initializes a dynamically allocated variable.
Insert	Inserts a substring at a specified point in a string.
Int	Returns the integer part of a real number.
IntToStr	Converts an integer to a string.
Length	Returns the length of a string or array.
Lo	Returns the low-order byte of an expression as an unsigned value.
Low	Returns the lowest value in the range of a type, array, or string.
LowerCase	Converts an ASCII string to lowercase.
MaxIntValue	Returns the largest signed value in an integer array.
MaxValue	Returns the largest signed value in an array.
MinIntValue	Returns the smallest signed value in an integer array.
MinValue	Returns smallest signed value in an array.
New	Creates a new dynamic variable and references it with a specified pointer.
Now	Returns the current date and time.
Ord	Returns the ordinal value of an ordinal-type expression.
Pos	Returns the index of the first character of a specified substring in a string.
Pred	Returns the predecessor of an ordinal value.
Ptr	Converts a specified address to a pointer.
Random	Generates random numbers within a specified range.
ReallocMem	Reallocates a dynamic variable.
Round	Returns the value of a real rounded to the nearest whole number.
SetLength	Sets the dynamic length of a string variable or array.
SetString	Sets the contents and length of the given string.
ShowException	Displays an exception message with its address.
ShowMessage	Displays a message box with an unformatted string and an OK button.
ShowMessageFmt	Displays a message box with a formatted string and an OK button.
Sin	Returns the sine of an angle in radians.
SizeOf	Returns the number of bytes occupied by a variable or type.
Sqr	Returns the square of a number.
Sqrt	Returns the square root of a number.
Str	Formats a string and returns it to a variable.
StrToCurr	Converts a string to a currency value.
StrToDate	Converts a string to a date format (TDateTime).
StrToDateTime	Converts a string to a TDateTime.
StrToFloat	Converts a string to a floating-point value.
StrToInt	Converts a string to an integer.
StrToTime	Converts a string to a time format (TDateTime).
StrUpper	Returns a string in upper case.
Succ	Returns the successor of an ordinal value.
Sum	Returns the sum of the elements from an array.
Time	Returns the current time.
TimeToStr	Converts a variable of type TDateTime to a string.
Trunc	Truncates a real number to an integer.
UniqueString	Ensures that a string has only one reference. (The string may be copied to produce a single reference.)
UpCase	Converts a character to uppercase.
UpperCase	Returns a string in uppercase.
VarArrayCreate	Creates a variant array.
VarArrayDimCount	Returns number of dimensions of a variant array.
VarARrayHighBound	Returns high bound for a dimension in a variant array.
VarArrayLock	Locks a variant array and returns a pointer to the data.
VarArrayLowBound	Returns the low bound of a dimension in a variant array.
VarArrayOf	Creates and fills a one-dimensional variant array.
VarArrayRedim	Resizes a variant array.
VarArrayRef	Returns a reference to the passed variant array.
VarArrayUnlock	Unlocks a variant array.
VarAsType	Converts a variant to specified type.
VarCast	Converts a variant to a specified type, storing the result in a variable.
VarClear	Clears a variant.
VarCopy	Copies a variant.
VarToStr	Converts variant to string.
VarType	Returns type code of specified variant.
For information on format strings, see Format strings.


Libraries and packages: Overview
  Object Pascal Language Guide

Libraries and packages

Topic groups	See also

A dynamically loadable library is a dynamic-link library (DLL) on Windows or a shared object library file on Linux. It is a collection of routines that can be called by applications and by other DLLs or shared objects. Like units, dynamically loadable libraries contain sharable code or resources. But this type of library is a separately compiled executable that is linked at runtime to the programs that use it.

To distinguish them from standalone executables, on Windows files containing compiled DLLs are named with the .DLL extension. On Linux, files containing shared object files are named with a .so extension. Object Pascal programs can call DLLs or shared objects written in other languages, and applications written in other languages can call DLLs or shared objects written in Object Pascal.

Calling DLLs or shared objects

Writing DLLs or shared objects

Packages


Calling dynamically loadable libraries
  Object Pascal Language Guide

Calling dynamically loadable libraries

Topic groups	See also

You can call operating system routines directly, but they are not linked to your application until runtime. This means that the library need not be present when you compile your program. It also means that there is no compile-time validation of attempts to import a routine.

Before you can call routines defined in a shared object, you must import them. This can be done in two ways: by declaring an external procedure or function, or by direct calls to the operating system. Whichever method you use, the routines are not linked to your application until runtime.

Object Pascal does not support importing of variables from shared libraries.

Static loading

The simplest way to import a procedure or function is to declare it using the external directive. For example,

On Windows:procedure DoSomething; external 'MYLIB.DLL';

On Linux:  procedure DoSomething; external 'mylib.so';

If you include this declaration in a program, MYLIB.DLL (Windows) or mylib.so (Linux) is loaded once, when the program starts. Throughout execution of the program, the identifier DoSomething always refers to the same entry point in the same shared library.

Declarations of imported routines can be placed directly in the program or unit where they are called. To simplify maintenance, however, you can collect external declarations into a separate “import unit” that also contains any constants and types required for interfacing with the library. Other modules that use the import unit can call any routines declared in it.

For more information about external declarations, see External declarations.


Dynamic loading
  Object Pascal Language Guide

Dynamic loading

Topic groups	See also

You can access routines in a library through direct calls to OS library functions, including LoadLibrary, FreeLibrary, and GetProcAddress. In Windows, these functions are declared in Windows.pas; on Linux, they are implemented for compatibility in SysUtils.pas; the actual Linux OS routines are dlopen, dlclose, and dlsym (all declared in Kylix’s Libc unit; see the man pages for more information). In this case, use procedural-type variables to reference the imported routines.

For example, on Windows or Linux:

uses Windows, ...; {On Linux, replace Windows with SysUtils }

type

  TTimeRec = record

    Second: Integer;

    Minute: Integer;

    Hour: Integer;

  end;

  TGetTime = procedure(var Time: TTimeRec);

  THandle = Integer;

var

  Time: TTimeRec;

  Handle: THandle;

  GetTime: TGetTime;

  ...

begin

  Handle := LoadLibrary('libraryname');

  if Handle <> 0 then

  begin

    @GetTime := GetProcAddress(Handle, 'GetTime');

    if @GetTime <> nil then

    begin

      GetTime(Time);

      with Time do

        WriteLn('The time is ', Hour, ':', Minute, ':', Second);

    end;

    FreeLibrary(Handle);

  end;

end;

When you import routines this way, the library is not loaded until the code containing the call to LoadLibrary executes. The library is later unloaded by the call to FreeLibrary. This allows you to conserve memory and to run your program even when some of the libraries it uses are not present.

This same example can also be written on Linux as follows:

uses Libc, ...;

type

  TTimeRec = record

    Second: Integer;

    Minute: Integer;

    Hour: Integer;

  end;

  TGetTime = procedure(var Time: TTimeRec);

  THandle = Pointer;

var

  Time: TTimeRec;

  Handle: THandle;

  GetTime: TGetTime;

  ...

begin

  Handle := dlopen('datetime.so', RTLD_LAZY);

  if Handle <> 0 then

  begin

    @GetTime := dlsym(Handle, 'GetTime');

    if @GetTime <> nil then

    begin

      GetTime(Time);

      with Time do

        WriteLn('The time is ', Hour, ':', Minute, ':', Second);

    end;

    dlclose(Handle);

  end;

end;

In this case, when importing routines, the shared object is not loaded until the code containing the call to dlopen executes. The shared object is later unloaded by the call to dlclose. This also allows you to conserve memory and to run your program even when some of the shared objects it uses are not present.


Writing dynamically loadable libraries
  Object Pascal Language Guide

Writing dynamically loadable libraries

Topic groups	See also

The main source for a dynamically loadable library is identical to that of a program, except that it begins with the reserved word library (instead of program).

Only routines that a library explicitly exports are available for importing by other libraries or programs. The following example shows a library with two exported functions, Min and Max.

library MinMax;

function Min(X, Y: Integer): Integer; stdcall;

begin

  if X < Y then Min := X else Min := Y;

end;

function Max(X, Y: Integer): Integer; stdcall;

begin

  if X > Y then Max := X else Max := Y;

end;

exports

  Min,

  Max;

begin

end.

If you want your library to be available to applications written in other languages, it’s safest to specify stdcall in the declarations of exported functions. Other languages may not support Object Pascal’s default register calling convention.

Libraries can be built from multiple units. In this case, the library source file is frequently reduced to a uses clause, an exports clause, and the initialization code. For example,

library Editors;

uses EdInit, EdInOut, EdFormat, EdPrint;

exports

  InitEditors,

  DoneEditors name Done,

  InsertText name Insert,

  DeleteSelection name Delete,

  FormatSelection,

  PrintSelection name Print,

   ...

  SetErrorHandler;

begin

  InitLibrary;

end.

You can put exports clauses in the interface or implementation section of a unit. Any library that includes such a unit in its uses clause automatically exports the routines listed the unit’s exports clauses—without the need for an exports clause of its own.

The directive local, which marks routines as unavailable for export, is platform-specific and has no effect in Windows programming.

On Linux, the local directive provides a slight performance optimization for routines that are compiled into a library but are not exported. This directive can be specified for standalone procedures and functions, but not for methods. A routine declared with local—for example,

function Contraband(I: Integer): Integer; local;

—does not refresh the EBX register and hence

cannot be exported from a library.

cannot be declared in the interface section of a unit.

cannot have its address taken or be assigned to a procedural-type variable.

if it is a pure assembler routine, cannot be called from another unit unless the caller sets up EBX.

The exports clause

Library initialization code

Global variables in a library

Libraries and system variables

Exceptions and runtime errors in libraries

The shared-memory manager (Windows only)


The exports clause
  Object Pascal Language Guide

The exports clause

Topic groups	See also

A routine is exported when it is listed in an exports clause, which has the form

exports entry1, ..., entryn;

where each entry consists of the name of a procedure, function, or variable (which must be declared prior to the exports clause), followed by a parameter list (only if exporting a routine that is overloaded), and an optional name specifier. You can qualify the procedure or function name with the name of a unit.

(Entries can also include the directive resident, which is maintained for backward compatibility and is ignored by the compiler.)

On Windows only, an index specifier consists of the directive index followed by a numeric constant between 1 and 2,147,483,647. (For more efficient programs, use low index values.) If an entry has no index specifier, the routine is automatically assigned a number in the export table.

Note:	Use of index specifiers, which are supported for backward compatibility only, is discouraged and may cause problems for other development tools.

A name specifier consists of the directive name followed by a string constant. If an entry has no name specifier, the routine is exported under its original declared name, with the same spelling and case. Use a name clause when you want to export a routine under a different name. For example,

exports

  DoSomethingABC name 'DoSomething';

When you export an overloaded function or procedure from a dynamically loadable library, you must specify its parameter list in the exports clause. For example,

exports

  Divide(X, Y: Integer) name 'Divide_Ints',

  Divide(X, Y: Real) name 'Divide_Reals';

On Windows, do not include index specifiers in entries for overloaded routines.

An exports clause can appear anywhere and any number of times in the declaration part of a program or library, or in the interface or implementation section of a unit. Programs seldom contain an exports clause.


Library initialization code
  Object Pascal Language Guide

Library initialization code

Topic groups	See also

The statements in a library’s block constitute the library’s initialization code. These statements are executed once every time the library is loaded. They typically perform tasks like registering window classes and initializing variables. Library initialization code can also install an exit procedure using the ExitProc variable, as described in Exit procedures; the exit procedure executes when the library is unloaded.

Library initialization code can signal an error by setting the ExitCode variable to a nonzero value. ExitCode is declared in the System unit and defaults to zero, indicating successful initialization. If a library’s initialization code sets ExitCode to another value, the library is unloaded and the calling application is notified of the failure. Similarly, if an unhandled exception occurs during execution of the initialization code, the calling application is notified of a failure to load the library.

Here is an example of a library with initialization code and an exit procedure.

library Test;

var

  SaveExit: Pointer;

procedure LibExit;

begin

  ...  // library exit code

  ExitProc := SaveExit;  // restore exit procedure chain

end;

begin

  ...  // library initialization code

  SaveExit := ExitProc;  // save exit procedure chain

  ExitProc := @LibExit;  // install LibExit exit procedure

end.

When a library is unloaded, it’s exit procedures are executed by repeated calls to the address stored in ExitProc, until ExitProc becomes nil. The initialization parts of all units used by a library are executed before the library’s initialization code, and the finalization parts of those units are executed after the library’s exit procedure.


Global variables in a library
  Object Pascal Language Guide

Global variables in a library

Topic groups	See also

Global variables declared in a shared library cannot be imported by an Object Pascal application.

A library can be used by several applications at once, but each application has a copy of the library in its own process space with its own set of global variables. For multiple libraries—or multiple instances of a library—to share memory, they must use memory-mapped files. Refer to the your system documentation for further information.


Libraries and system variables
  Object Pascal Language Guide

Libraries and system variables

Topic groups	See also

Several variables declared in the System unit are of special interest to those programming libraries. Use IsLibrary to determine whether code is executing in an application or in a library; IsLibrary is always False in an application and True in a library. During a library’s lifetime, HInstance contains its instance handle. CmdLine is always nil in a library.

The DLLProc variable allows a library to monitor calls that the operating system makes to the library entry point. This feature is normally used only by libraries that support multithreading. DLLProc is available on both Windows and Linux but its use differs on each. On Windows, DLLProc is used in multithreading applications; on Linux, it is used to determine when your library is being unloaded. You should use finalization sections, rather than exit procedures, for all exit behavior.

To monitor operating-system calls, create a callback procedure that takes a single integer parameter—for example,

procedure DLLHandler(Reason: Integer);

—and assign the address of the procedure to the DLLProc variable. When the procedure is called, it passes to it one of the following values.

DLL_PROCESS_DETACH	Indicates that the library is detaching from the address space of the calling process as a result of a clean exit or a call to FreeLibrary or (dlclose on Linux).
DLL_THREAD_ATTACH	Indicates that the current process is creating a new thread (Windows only).
DLL_THREAD_DETACH	Indicates that a thread is exiting cleanly (Windows only).
On Linux, these are defined in the Libc unit.

In the body of the procedure, you can specify actions to take depending on which parameter is passed to the procedure.


Exceptions and runtime errors in libraries
  Object Pascal Language Guide

Exceptions and runtime errors in libraries

Topic groups	See also

When an exception is raised but not handled in a dynamically loadable library, it propagates out of the library to the caller. If the calling application or library is itself written in Object Pascal, the exception can be handled through a normal try...except statement.

Note:	Under Linux this is only possible if the library and application have both been built with the same set of (base) runtime packages (which contains the EH code) or if both link to ShareExcept.

If the calling application or library is written in another language, the exception can be handled as an operating-system exception with the exception code $0EEDFACE. The first entry in the ExceptionInformation array of the operating-system exception record contains the exception address, and the second entry contains a reference to the Object Pascal exception object.

Generally, you should not let exceptions escape from your library. On Windows, Delphi exceptions map to the OS exception model; Linux does not have an exception model.

If a library does not use the SysUtils unit, exception support is disabled. In this case, when a runtime error occurs in the library, the calling application terminates. Because the library has no way of knowing whether it was called from an Object Pascal program, it cannot invoke the application’s exit procedures; the application is simply aborted and removed from memory.


Shared-memory manager (Windows only)
  Object Pascal Language Guide

Shared-memory manager (Windows only)

Topic groups	See also

On Windows, if a DLL exports routines that pass long strings or dynamic arrays as parameters or function results (whether directly or nested in records or objects), then the DLL and its client applications (or DLLs) must all use the ShareMem unit. The same is true if one application or DLL allocates memory with New or GetMem which is deallocated by a call to Dispose or FreeMem in another module. ShareMem should always be the first unit listed in any program or library uses clause where it occurs.

ShareMem is the interface unit for the BORLANDMM.DLL memory manager, which allows modules to share dynamically allocated memory. BORLANDMM.DLL must be deployed with applications and DLLs that use ShareMem. When an application or DLL uses ShareMem, its memory manager is replaced by the memory manager in BORLANDMM.DLL.

Linux uses glibc’s malloc to manage shared memory.


Packages: Overview
  Object Pascal Language Guide

Packages

Topic groups	See also

A package is a specially compiled library used by applications, the IDE, or both. Packages allow you to rearrange when code resides without affecting the source code. This is sometimes referred to as application partitioning.

Runtime packages provide functionality when a user runs an application. Design-time packages are used to install components in the IDE and to create special property editors for custom components. A single package can function at both design time and runtime, and design-time packages frequently work by referencing runtime packages in their requires clauses.

To distinguish them from other libraries, packages are stored in files:

On Windows, package files end with the .bpl (Borland package library) extension.

On Linux, packages generally begin with the prefix bpl and have a .so extension.

Ordinarily, packages are loaded statically when an applications starts. But you can use the LoadPackage and UnloadPackage routines (in the SysUtils unit) to load packages dynamically.

Note:	When an application utilizes packages, the name of each packaged unit still must appear in the uses clause of any source file that references it.

Package declarations and source files

Compiling packages


Package declarations and source files
  Object Pascal Language Guide

Package declarations and source files

Topic groups	See also

Each package is declared in a separate source file, which should be saved with the .dpk extension to avoid confusion with other files containing Object Pascal code. A package source file does not contain type, data, procedure, or function declarations. Instead, it contains

A name for the package.

A list of other packages required by the new package. These are packages to which the new package is linked.

A list of unit files contained by, or bound into, the package when it is compiled. The package is essentially a wrapper for these source-code units, which provide the functionality of the compiled package.

A package declaration has the form

package packageName;

  requiresClause;

  containsClause;

end.

where packageName is any valid identifier. The requiresClause and containsClause are both optional. For example, the following code declares the DATAX package.

package DATAX;

requires

  baseclx,

  visualclx;

  contains Db, DBLocal, DBXpress, ... ;

end.

The requires clause lists other, external packages used by the package being declared. It consists of the directive requires, followed by a comma-delimited list of package names, followed by a semicolon. If a package does not reference other packages, it does not need a requires clause.

The contains clause identifies the unit files to be compiled and bound into the package. It consists of the directive contains, followed by a comma-delimited list of unit names, followed by a semicolon. Any unit name may be followed by the reserved word in and the name of a source file, with or without a directory path, in single quotation marks; directory paths can be absolute or relative. For example,

contains MyUnit in 'C:\MyProject\MyUnit.pas'; // Windows

contains MyUnit in '\home\developer\MyProject\MyUnit.pas'; // Linux

Note:	Thread-local variables (declared with threadvar) in a packaged unit cannot be accessed from clients that use the package.

Naming packages

The requires clause

The contains clause


Naming packages
  Object Pascal Language Guide

Naming packages

Topic groups	See also

A compiled package involves several generated files. For example, the source file for the package called DATAX is DATAX.dpk, from which the compiler generates an executable and a binary image called

On Windows: DATAX.bpl and DATAX.dcp

On Linux: bplDATAX.so and DATAX.dcp.

DATAX is used to refer to the package in the requires clauses of other packages, or when using the package in an application. Package names must be unique within a project.


The requires clause
  Object Pascal Language Guide

The requires clause

Topic groups	See also

The requires clause lists other, external packages that are used by the current package. It functions like the uses clause in a unit file. An external package listed in the requires clause is automatically linked at compile time into any application that uses both the current package and one of the units contained in the external package.

If the unit files contained in a package make references to other packaged units, the other packages should be included in the first package’s requires clause. If the other packages are omitted from the requires clause, the compiler loads the referenced units from their .dcu (Windows) or .dpu (Linux) files.

Avoiding circular package references

Packages cannot contain circular references in their requires clauses. This means that

A package cannot reference itself in its own requires clause.

A chain of references must terminate without rereferencing any package in the chain. If package A requires package B, then package B cannot require package A; if package A requires package B and package B requires package C, then package C cannot require package A.

Duplicate package references

The compiler ignores duplicate references in a package’s requires clause. For programming clarity and readability, however, duplicate references should be removed.


The contains clause
  Object Pascal Language Guide

The contains clause

Topic groups	See also

The contains clause identifies the unit files to be bound into the package. Do not include file-name extensions in the contains clause.

Avoiding redundant source code uses

A package cannot be listed in the contains clause of another package or the uses clause of a unit.

All units included directly in a package’s contains clause, or indirectly in the uses clauses of those units, are bound into the package at compile time. The units contained (directly or indirectly) in a package cannot be contained in any other packages referenced in requires clause of that package.

A unit cannot be contained (directly or indirectly) in more than one package used by the same application.


Compiling packages: Overview
  Object Pascal Language Guide

Compiling packages

Topic groups	See also

Packages are ordinarily compiled from the IDE using .dpk files generated by the Package editor. You can also compile .dpk files directly from the command line. When you build a project that contains a package, the package is implicitly recompiled, if necessary.

Generated files

Package-specific compiler directives

Package-specific command-line compiler switches


Generated files
  Object Pascal Language Guide

Generated files

Topic groups	See also

The following table lists the files produced by the successful compilation of a package.

File extension	Contents
dcp	A binary image containing a package header and the concatenation of all dcu (Windows) or dpu (Linux) files in the package. A single dcp file is created for each package. The base name for the dcp is the base name of the dpk source file.
dcu (Windows) dpu (Linux)	A binary image for a unit file contained in a package. One dcu or dpu file is created, when necessary, for each unit file.
.bpl on Windows bpl<package>.so  on Linux	The runtime package. This file is a shared library with special Borland-specific features. The base name for the package is the base name of the dpk source file.


Package-specific compiler directives
  Object Pascal Language Guide

Package-specific compiler directives

Topic groups	See also

The following table lists package-specific compiler directives that can be inserted into source code.

Directive	Purpose
{$IMPLICITBUILD OFF}	Prevents a package from being implicitly recompiled later. Use in .dpk files when compiling packages that provide low-level functionality, that change infrequently between builds, or whose source code will not be distributed.
{$G–} or {$IMPORTEDDATA OFF}	Disables creation of imported data references. This directive increases memory-access efficiency, but prevents the unit where it occurs from referencing variables in other packages.
{$WEAKPACKAGEUNIT ON}	Packages unit weakly.
{$DENYPACKAGEUNIT ON}	Prevents unit from being placed in a package.
{$DESIGNONLY ON}	Compiles the package for installation in the IDE. (Put in .dpk file.)
{$RUNONLY ON}	Compiles the package as runtime only. (Put in .dpk file.)
Including {$DENYPACKAGEUNIT ON} in source code prevents the unit file from being packaged. Including {$G–} or {$IMPORTEDDATA OFF} may prevent a package from being used in the same application with other packages.

Other compiler directives may be included, if appropriate, in package source code.


Package-specific command-line compiler switches
  Object Pascal Language Guide

Package-specific command-line compiler switches

Topic groups	See also

The following package-specific switches are available for the command-line compiler.

Switch	Purpose
–$G–	Disables creation of imported data references. Using this switch increases memory-access efficiency, but prevents packages compiled with it from referencing variables in other packages.
–LE path	Specifies the directory where the compiled package file will be placed.
–LN path	Specifies the directory where the package dcp file will be placed.
–LUpackageName [;packageName2;...]	Specifies additional runtime packages to use in an application. Used when compiling a project.
–Z	Prevents a package from being implicitly recompiled later. Use when compiling packages that provide low-level functionality, that change infrequently between builds, or whose source code will not be distributed.
Using the –$G– switch may prevent a package from being used in the same application with other packages.

Other command-line options may be used, if appropriate, when compiling packages.


Object interfaces: Overview
  Object Pascal Language Guide

Object interfaces

Topic groups	See also

An object interface—or simply interface—defines methods that can be implemented by a class. Interfaces are declared like classes, but cannot be directly instantiated and do not have their own method definitions. Rather, it is the responsibility of any class that supports an interface to provide implementations for the interface’s methods. A variable of an interface type can reference an object whose class implements that interface; however, only methods declared in the interface can be called using such a variable.

Interfaces offer some of the advantages of multiple inheritance without the semantic difficulties. They are also essential for using distributed object models. Custom objects built that support interfaces can interact with objects written in C++, Java, and other languages.

Interface types

Implementing interfaces

Interface references

Automation objects


Interface types: Overview
  Object Pascal Language Guide

Interface types

Topic groups	See also

Interfaces, like classes, can be declared only in the outermost scope of a program or unit, not in a procedure or function declaration. An interface type declaration has the form

type interfaceName = interface (ancestorInterface)

  ['{GUID}']

  memberList

end;

where (ancestorInterface) and ['{GUID}'] are optional. In most respects, interface declarations resemble class declarations, but the following restrictions apply.

The memberList can include only methods and properties. Fields are not allowed in interfaces.

Since an interface has no fields, property read and write specifiers must be methods.

All members of an interface are public. Visibility specifiers and storage specifiers are not allowed. (But an array property can be declared as default.)

Interfaces have no constructors or destructors. They cannot be instantiated, except through classes that implement their methods.

Methods cannot be declared as virtual, dynamic, abstract, or override. Since interfaces do not implement their own methods, these designations have no meaning.

Here is an example of an interface declaration:

type

  IMalloc = interface(IInterface)

    ['{00000002-0000-0000-C000-000000000046}']

    function Alloc(Size: Integer): Pointer; stdcall;

    function Realloc(P: Pointer; Size: Integer): Pointer; stdcall;

    procedure Free(P: Pointer); stdcall;

    function GetSize(P: Pointer): Integer; stdcall;

    function DidAlloc(P: Pointer): Integer; stdcall;

    procedure HeapMinimize; stdcall;

  end;

In some interface declarations, the interface reserved word is replaced by dispinterface. This construction (along with the dispid, readonly, and writeonly directives) is platform-specific and is not used in Linux programming.

IUnknown and inheritance

Interface identification

Calling conventions

Interface properties

Forward declarations


IInterface and inheritance
  Object Pascal Language Guide

IInterface and inheritance

Topic groups	See also

An interface, like a class, inherits all of its ancestors’ methods. But interfaces, unlike classes, do not implement methods. What an interface inherits is the obligation to implement methods—an obligation that devolves onto any class supporting the interface.

The declaration of an interface can specify an ancestor interface. If no ancestor is specified, the interface is a direct descendant of IInterface, which is defined in the System unit and is the ultimate ancestor of all other interfaces. IInterface declares three methods: QueryInterface, _AddRef, and _Release.

Note:	IInterface is equivalent to IUnknown. You should generally use IInterface for platform independent applications and reserve the use of IUnknown for specific programs that include Windows dependencies.

QueryInterface provides the means to move freely among the different interfaces that an object supports. _AddRef and _Release provide lifetime memory management for interface references. The easiest way to implement these methods is to derive the implementing class from the System unit’s TInterfacedObject. It is also possible to dispense with any of these methods by implementing it as an empty function; COM objects (Windows only), however, must be managed through _AddRef and _Release.


Interface identification
  Object Pascal Language Guide

Interface identification

Topic groups	See also

An interface declaration can specify a globally unique identifier (GUID), represented by a string literal enclosed in brackets immediately preceding the member list. The GUID part of the declaration must have the form

['{xxxxxxxx–xxxx–xxxx–xxxx–xxxxxxxxxxxx}']

where each x is a hexadecimal digit (0 through 9 or A through F). On Windows, the Type Library editor automatically generates GUIDs for new interfaces; you can also generate GUIDs by pressing Ctrl+Shift+G in the Code editor (on Linux, you must use Ctrl+Shift+G).

A GUID is a 16-byte binary value that uniquely identifies an interface. If an interface has a GUID, you can use interface querying to get references to its implementations.

The TGUID and PGUID types, declared in the System unit, are used to manipulate GUIDs.

type

  PGUID = ^TGUID;

  TGUID = packed record

    D1: Longword;

    D2: Word;

    D3: Word;

    D4: array[0..7] of Byte;

  end;

When you declare a typed constant of type TGUID, you can use a string literal to specify its value. For example,

const IID_IMalloc: TGUID = '{00000002-0000-0000-C000-000000000046}';

In procedure and function calls, either a GUID or an interface identifier can serve as a value or constant parameter of type TGUID. For example, given the declaration

function Supports(Unknown: IInterface; const IID: TGUID): Boolean;

Supports can be called in either of two ways:

if Supports(Allocator, IMalloc) then ...

if Supports(Allocator, IID_IMalloc) then ...


Calling conventions for interfaces
  Object Pascal Language Guide

Calling conventions for interfaces

Topic groups	See also

The default calling convention is register, but interfaces shared among modules (especially if they are written in different languages) should declare all methods with stdcall. Use safecall to implement CORBA interfaces. On Windows, you can use safecall to implement methods of dual interfaces .

For more information about calling conventions, see Calling conventions.


Interface properties
  Object Pascal Language Guide

Interface properties

Topic groups	See also

Properties declared in an interface are accessible only through expressions of the interface type; they cannot be accessed through class-type variables. Moreover, interface properties are visible only within programs where the interface is compiled. For example, on Windows, COM objects do not have properties.

In an interface, property read and write specifiers must be methods, since fields are not available.


Forward declarations
  Object Pascal Language Guide

Forward declarations

Topic groups	See also

An interface declaration that ends with the reserved word interface and a semicolon, without specifying an ancestor, GUID, or member list, is a forward declaration. A forward declaration must be resolved by a defining declaration of the same interface within the same type declaration section. In other words, between a forward declaration and its defining declaration, nothing can occur except other type declarations.

Forward declarations allow mutually dependent interfaces. For example,

type

  IControl = interface;

  IWindow = interface

    ['{00000115-0000-0000-C000-000000000044}']

    function GetControl(Index: Integer): IControl;

    ...

  end;

  IControl = interface

    ['{00000115-0000-0000-C000-000000000049}']

    function GetWindow: IWindow;

    ...

  end;

Mutually derived interfaces are not allowed. For example, it is not legal to derive IWindow from IControl and also derive IControl from IWindow.


Implementing interfaces
  Object Pascal Language Guide

Implementing interfaces

Topic groups	See also

Once an interface has been declared, it must be implemented in a class before it can be used. The interfaces implemented by a class are specified in the class’s declaration, after the name of the class’s ancestor. Such declarations have the form

type className = class (ancestorClass, interface1, ..., interfacen)

  memberList

 end;

For example,

type

  TMemoryManager = class(TInterfacedObject, IMalloc, IErrorInfo)

    ...

  end;

declares a class called TMemoryManager that implements the IMalloc and IErrorInfo interfaces. When a class implements an interface, it must implement (or inherit an implementation of) each method declared in the interface.

Here is the declaration of TInterfacedObject in the System unit.

type

TInterfacedObject = class(TObject, IInterface)

protected

    FRefCount: Integer;

    function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;

    function _AddRef: Integer; stdcall;

    function _Release: Integer; stdcall;

  public

    procedure AfterConstruction; override;

    procedure BeforeDestruction; override;

    class function NewInstance: TObject; override;

    property RefCount: Integer read FRefCount;

  end;

TInterfacedObject implements the IInterface interface. Hence TInterfacedObject declares and implements each of IInterface’s three methods.

Classes that implement interfaces can also be used as base classes. (The first example above declares TMemoryManager as a direct descendent of TInterfacedObject.) Since every interface inherits from IInterface, a class that implements interfaces must implement the QueryInterface, _AddRef, and _Release methods. The System unit’s TInterfacedObject implements these methods and is thus a convenient base from which to derive other classes that implement interfaces.

When an interface is implemented, each of its methods is mapped onto a method in the implementing class that has the same result type, the same calling convention, the same number of parameters, and identically typed parameters in each position. By default, each interface method is mapped to a method of the same name in the implementing class.

Method resolution clauses

Changing inherited implementations

Implementing interfaces by delegation


Method resolution clauses
  Object Pascal Language Guide

Method resolution clauses

Topic groups	See also

You can override the default name-based mappings by including method resolution clauses in a class declaration. When a class implements two or more interfaces that have identically named methods, use method resolution clauses to resolve the naming conflicts.

A method resolution clause has the form

procedure interface.interfaceMethod = implementingMethod;

or

function interface.interfaceMethod = implementingMethod;

where implementingMethod is a method declared in the class or one of its ancestors. The implementingMethod can be a method declared later in the class declaration, but cannot be a private method of an ancestor class declared in another module.

For example, the class declaration

type

  TMemoryManager = class(TInterfacedObject, IMalloc, IErrorInfo)

    function IMalloc.Alloc = Allocate;

    procedure IMalloc.Free = Deallocate;

    ...

  end;

maps IMalloc’s Alloc and Free methods onto TMemoryManager’s Allocate and Deallocate methods.

A method resolution clause cannot alter a mapping introduced by an ancestor class.


Changing inherited implementations
  Object Pascal Language Guide

Changing inherited implementations

Topic groups	See also

Descendant classes can change the way a specific interface method is implemented by overriding the implementing method. This requires that the implementing method be virtual or dynamic.

A class can also reimplement an entire interface that it inherits from an ancestor class. This involves relisting the interface in the descendant class’s declaration. For example,

type

  IWindow = interface

    ['{00000115-0000-0000-C000-000000000146}']

    procedure Draw;

    ...

  end;

  TWindow = class(TInterfacedObject, IWindow)  // TWindow implements IWindow

    procedure Draw;

    ...

  end;

  TFrameWindow = class(TWindow, IWindow)  // TFrameWindow reimplements IWindow

    procedure Draw;

    ...

  end;

Reimplementing an interface hides the inherited implementation of the same interface. Hence method resolution clauses in an ancestor class have no effect on the reimplemented interface.


Implementing interfaces by delegation
  Object Pascal Language Guide

Implementing interfaces by delegation

Topic groups	See also

The implements directive allows you to delegate implementation of an interface to a property in the implementing class. For example,

property MyInterface: IMyInterface read FMyInterface implements IMyInterface;

declares a property called MyInterface that implements the interface IMyInterface.

The implements directive must be the last specifier in the property declaration and can list more than one interface, separated by commas. The delegate property

must be of a class or interface type.

cannot be an array property or have an index specifier.

must have a read specifier. If the property uses a read method, that method must use the default register calling convention and cannot be dynamic (though it can be virtual) or specify the message directive.

Note:	The class you use to implement the delegated interface should derive from TAggregatedObject.

Delegating to an interface-type property

Delegating to a class-type property


Delegating to an interface-type property
  Object Pascal Language Guide

Delegating to an interface-type property

Topic groups	See also

If the delegate property is of an interface type, that interface, or an interface from which it derives, must occur in the ancestor list of the class where the property is declared. The delegate property must return an object whose class completely implements the interface specified by the implements directive, and which does so without method resolution clauses. For example,

type

  IMyInterface = interface

    procedure P1;

    procedure P2;

  end;

  TMyClass = class(TObject, IMyInterface)

    FMyInterface: IMyInterface;

    property MyInterface: IMyInterface read FMyInterface implements IMyInterface;

  end;

var

  MyClass: TMyClass;

  MyInterface: IMyInterface;

begin

  MyClass := TMyClass.Create;

  MyClass.FMyInterface := ...  // some object whose class implements IMyInterface

  MyInterface := MyClass;

  MyInterface.P1;

end;


Delegating to a class-type property
  Object Pascal Language Guide

Delegating to a class-type property

Topic groups	See also

If the delegate property is of a class type, that class and its ancestors are searched for methods implementing the specified interface before the enclosing class and its ancestors are searched. Thus it is possible to implement some methods in the class specified by the property, and others in the class where the property is declared. Method resolution clauses can be used in the usual way to resolve ambiguities or specify a particular method. An interface cannot be implemented by more than one class-type property. For example,

type

  IMyInterface = interface

    procedure P1;

    procedure P2;

  end;

  TMyImplClass = class

    procedure P1;

    procedure P2;

  end;

  TMyClass = class(TInterfacedObject, IMyInterface)

    FMyImplClass: TMyImplClass;

    property MyImplClass: TMyImplClass read FMyImplClass implements IMyInterface;

    procedure IMyInterface.P1 = MyP1;

    procedure MyP1;

  end;

procedure TMyImplClass.P1;

  ...

procedure TMyImplClass.P2;

  ...

procedure TMyClass.MyP1;

  ...

var

  MyClass: TMyClass;

  MyInterface: IMyInterface;

begin

  MyClass := TMyClass.Create;

  MyClass.FMyImplClass := TMyImplClass.Create;

  MyInterface := MyClass;

  MyInterface.P1;        // calls TMyClass.MyP1;

  MyInterface.P2;        // calls TImplClass.P2;

end;


Interface references
  Object Pascal Language Guide

Interface references

Topic groups	See also

If you declare a variable of an interface type, the variable can reference instances of any class that implements the interface. Such variables allow you to call interface methods without knowing at compile time where the interface is implemented. But they are subject to the following limitations.

An interface-type expression gives you access only to methods and properties declared in the interface, not to other members of the implementing class.

An interface-type expression cannot reference an object whose class implements a descendant interface, unless the class (or one that it inherits from) explicitly implements the ancestor interface as well.

For example,

type

  IAncestor = interface

  end;

  IDescendant = interface(IAncestor)

    procedure P1;

  end;

  TSomething = class(TInterfacedObject, IDescendant)

    procedure P1;

    procedure P2;

  end;

  ...

var

  D: IDescendant;

  A: IAncestor;

begin

  D := TSomething.Create;  // works!

  A := TSomething.Create;  // error

  D.P1;  // works!

  D.P2;  // error

end;

In this example,

A is declared as a variable of type IAncestor. Because TSomething does not list IAncestor among the interfaces it implements, a TSomething instance cannot be assigned to A. But if we changed TSomething’s declaration to

TSomething = class(TInterfacedObject, IAncestor, IDescendant)

 ...

the first error would become a valid assignment.

D is declared as a variable of type IDescendant. While D references an instance of TSomething, we cannot use it to access TSomething’s P2 method, since P2 is not a method of IDescendant. But if we changed D’s declaration to

D: TSomething;

the second error would become a valid method call.

Interface references are managed through reference-counting, which depends on the _AddRef and _Release methods inherited from IInterface. When an object is referenced only through interfaces, there is no need to destroy it manually; the object is automatically destroyed when the last reference to it goes out of scope.

Global interface-type variables can be initialized only to nil.

To determine whether an interface-type expression references an object, pass it to the standard function Assigned.

Interface assignment-compatibility

Interface typecasts

Interface querying


Interface assignment-compatibility
  Object Pascal Language Guide

Interface assignment-compatibility

Topic groups	See also

A class type is assignment-compatible with any interface type implemented by the class. An interface type is assignment-compatible with any ancestor interface type. The value nil can be assigned to any interface-type variable.

An interface-type expression can be assigned to a variant. If the interface is of type IDispatch or a descendant, the variant receives the type code varDispatch. Otherwise, the variant receives the type code varUnknown.

A variant whose type code is varEmpty, varUnknown, or varDispatch can be assigned to an IInterface variable. A variant whose type code is varEmpty or varDispatch can be assigned to an IDispatch variable.


Interface typecasts
  Object Pascal Language Guide

Interface typecasts

Topic groups	See also

Interface types follow the same rules as class types in variable and value typecasts. Class-type expressions can be cast to interface types—for example, IMyInterface(SomeObject)—provided the class implements the interface.

An interface-type expression can be cast to Variant. If the interface is of type IDispatch or a descendant, the resulting variant has the type code varDispatch. Otherwise, the resulting variant has the type code varUnknown.

A variant whose type code is varEmpty, varUnknown, or varDispatch can be cast to IInterface. A variant whose type code is varEmpty or varDispatch can be cast to IDispatch.

Interface querying


Interface querying
  Object Pascal Language Guide

Interface querying

Topic groups	See also

You can use the as operator to perform checked interface typecasts. This is known as interface querying, and it yields an interface-type expression from an object reference or from another interface reference, based on the actual (runtime) type of the object. An interface query has the form

object as interface

where object is an expression of an interface or variant type or denotes an instance of a class that implements an interface, and interface is any interface declared with a GUID.

An interface query returns nil if object is nil. Otherwise, it passes the GUID of interface to the QueryInterface method in object, raising an exception unless QueryInterface returns zero. If QueryInterface returns zero (indicating that object’s class implements interface), the interface query returns an interface reference to object.


Automation objects (Windows only)
  Object Pascal Language Guide

Automation objects (Windows only)

Topic groups	See also

An object whose class implements the IDispatch interface (declared in the System unit) is an Automation object. Automation is available on Windows only.

Dispatch interface types

Accessing Automation objects

Dual interfaces


Dispatch interface types (Windows only)
  Object Pascal Language Guide

Dispatch interface types (Windows only)

Topic groups	See also

Dispatch interface types define the methods and properties that an Automation object implements through IDispatch. Calls to methods of a dispatch interface are routed through IDispatch’s Invoke method at runtime; a class cannot implement a dispatch interface.

A dispatch interface type declaration has the form

type interfaceName = dispinterface

  ['{GUID}']

  memberList

end;

where ['{GUID}'] is optional and memberList consists of property and method declarations. Dispatch interface declarations are similar to regular interface declarations, but they cannot specify an ancestor. For example,

type

  IStringsDisp = dispinterface

    ['{EE05DFE2-5549-11D0-9EA9-0020AF3D82DA}']

    property ControlDefault[Index: Integer]: OleVariant dispid 0; default;

    function Count: Integer; dispid 1;

    property Item[Index: Integer]: OleVariant dispid 2;

    procedure Remove(Index: Integer); dispid 3;

    procedure Clear; dispid 4;

    function Add(Item: OleVariant): Integer; dispid 5;

    function _NewEnum: IUnknown; dispid -4;

  end;

Dispatch interface methods (Windows only)

Methods of a dispatch interface are prototypes for calls to the Invoke method of the underlying IDispatch implementation. To specify an Automation dispatch ID for a method, include the dispid directive in its declaration, followed by an integer constant; specifying an already used ID causes an error.

A method declared in a dispatch interface cannot contain directives other than dispid. Parameter and result types must be automatable—that is, they must be Byte, Currency, Real, Double, Longint, Integer, Single, Smallint, AnsiString, WideString, TDateTime, Variant, OleVariant, WordBool, or any interface type.

Dispatch interface properties

Properties of a dispatch interface do not include access specifiers. They can be declared as readonly or writeonly. To specify a dispatch ID for a property, include the dispid directive in its declaration, followed by an integer constant; specifying an already used ID causes an error. Array properties can be declared as default. No other directives are allowed in dispatch-interface property declarations.


Accessing Automation objects (Windows only)
  Object Pascal Language Guide

Accessing Automation objects (Windows only)

Topic groups	See also

Use variants to access Automation objects. When a variant references an Automation object, you can call the object’s methods and read or write to its properties through the variant. To do this, you must include ComObj in the uses clause of one of your units or your program or library.

Automation object method calls are bound at runtime and require no previous method declarations. The validity of these calls is not checked at compile time.

The following example illustrates Automation method calls. The CreateOleObject function (defined in ComObj) returns an IDispatch reference to an Automation object and is assignment-compatible with the variant Word.

var

  Word: Variant;

begin

  Word := CreateOleObject('Word.Basic');

  Word.FileNew('Normal');

  Word.Insert('This is the first line'#13);

  Word.Insert('This is the second line'#13);

  Word.FileSaveAs('c:\temp\test.txt', 3);

end;

You can pass interface-type parameters to Automation methods.

Variant arrays with an element type of varByte are the preferred method of passing binary data between Automation controllers and servers. Such arrays are subject to no translation of their data, and can be efficiently accessed using the VarArrayLock and VarArrayUnlock routines.

Automation object method-call syntax

The syntax of an Automation object method call or property access is similar to that of a normal method call or property access. Automation method calls, however, can use both positional and named parameters. (But some Automation servers do not support named parameters.)

A positional parameter is simply an expression. A named parameter consists of a parameter identifier, followed by the := symbol, followed by an expression. Positional parameters must precede any named parameters in a method call. Named parameters can be specified in any order.

Some Automation servers allow you to omit parameters from a method call, accepting their default values. For example,

Word.FileSaveAs('test.doc');

Word.FileSaveAs('test.doc', 6);

Word.FileSaveAs('test.doc',,,'secret');

Word.FileSaveAs('test.doc', Password := 'secret');

Word.FileSaveAs(Password := 'secret', Name := 'test.doc');

Automation method call parameters can be of integer, real, string, Boolean, and variant types. A parameter is passed by reference if the parameter expression consists only of a variable reference, and if the variable reference is of type Byte, Smallint, Integer, Single, Double, Currency, TDateTime, AnsiString, WordBool, or Variant. If the expression is not of one of these types, or if it is not just a variable, the parameter is passed by value. Passing a parameter by reference to a method that expects a value parameter causes COM to fetch the value from the reference parameter. Passing a parameter by value to a method that expects a reference parameter causes an error.


Dual interfaces (Windows only)
  Object Pascal Language Guide

Dual interfaces (Windows only)

Topic groups	See also

A dual interface is an interface that supports both compile-time binding and runtime binding through Automation. Dual interfaces must descend from IDispatch.

All methods of a dual interface (except from those inherited from IInterface and IDispatch) must use the safecall convention, and all method parameter and result types must be automatable. (The automatable types are Byte, Currency, Real, Double, Real48, Integer, Single, Smallint, AnsiString, ShortString, TDateTime, Variant, OleVariant, and WordBool.)


Memory management: Overview
  Object Pascal Language Guide

Memory management

Topic groups	See also

Note:	Linux uses glibc functions such as malloc for memory management. For information, refer to the malloc man page on your Linux system.

On Windows systems, the memory manager manages all dynamic memory allocations and deallocations in an application. The New, Dispose, GetMem, ReallocMem, and FreeMem standard procedures use the memory manager, and all objects and long strings are allocated through the memory manager.

On Windows, the memory manager is optimized for applications that allocate large numbers of small- to medium-sized blocks, as is typical for object-oriented applications and applications that process string data. Other memory managers, such as the implementations of GlobalAlloc, LocalAlloc, and private heap support in Windows, typically do not perform well in such situations, and would slow down an application if they were used directly.

To ensure the best performance, the memory manager interfaces directly with the Win32 virtual memory API (the VirtualAlloc and VirtualFree functions). The memory manager reserves memory from the operating system in 1-MB sections of address space, and commits memory as required in 16-KB increments. It decommits and releases unused memory in 16-KB and 1-MB sections. For smaller blocks, committed memory is further suballocated.

Memory manager blocks are always rounded upward to a 4-byte boundary, and always include a 4-byte header in which the size of the block and other status bits are stored. This means that memory manager blocks are always double-word-aligned, which guarantees optimal CPU performance when addressing the block.

The memory manager maintains two status variables, AllocMemCount and AllocMemSize, which contain the number of currently allocated memory blocks and the combined size of all currently allocated memory blocks. Applications can use these variables to display status information for debugging.

The System unit provides two procedures, GetMemoryManager and SetMemoryManager, that allow applications to intercept low-level memory manager calls. The System unit also provides a function called GetHeapStatus that returns a record containing detailed memory-manager status information.


Variables
  Object Pascal Language Guide

Variables

Topic groups	See also

Global variables are allocated on the application data segment and persist for the duration of the program. Local variables (declared within procedures and functions) reside in an application’s stack. Each time a procedure or function is called, it allocates a set of local variables; on exit, the local variables are disposed of. Compiler optimization may eliminate variables earlier.

Note:	On Linux, stack size is set by the environment only.

On Windows, an application’s stack is defined by two values: the minimum stack size and the maximum stack size. The values are controlled through the $MINSTACKSIZE and $MAXSTACKSIZE compiler directives, and default to 16,384 (16K) and 1,048,576 (1M) respectively. An application is guaranteed to have the minimum stack size available, and an application’s stack is never allowed to grow larger than the maximum stack size. If there is not enough memory available to satisfy an application’s minimum stack requirement, Windows will report an error upon attempting to start the application.

If a Windows application requires more stack space than specified by the minimum stack size, additional memory is automatically allocated in 4K increments. If allocation of additional stack space fails, either because more memory is not available or because the total size of the stack would exceed the maximum stack size, an EStackOverflow exception is raised. (Stack overflow checking is completely automatic. The $S compiler directive, which originally controlled overflow checking, is maintained for backward compatibility.)

On Windows or Linux, dynamic variables created with the GetMem or New procedure are heap-allocated and persist until they are deallocated with FreeMem or Dispose.

Long strings, wide strings, dynamic arrays, variants, and interfaces are heap-allocated, but their memory is managed automatically.


Integer types
  Object Pascal Language Guide

Integer types

Topic groups	See also

The format of an integer-type variable depends on its minimum and maximum bounds.

If both bounds are within the range –128..127 (Shortint), the variable is stored as a signed byte.

If both bounds are within the range 0..255 (Byte), the variable is stored as an unsigned byte.

If both bounds are within the range –32768..32767 (Smallint), the variable is stored as a signed word.

If both bounds are within the range 0..65535 (Word), the variable is stored as an unsigned word.

If both bounds are within the range –2147483648..2147483647 (Longint), the variable is stored as a signed double word.

If both bounds are within the range 0..4294967295 (Longword), the variable is stored as an unsigned double word.

Otherwise, the variable is stored as a signed quadruple word (Int64).


Character types
  Object Pascal Language Guide

Character types

Topic groups	See also

A Char, an AnsiChar, or a subrange of a Char type is stored as an unsigned byte. A WideChar is stored as an unsigned word.


Boolean types
  Object Pascal Language Guide

Boolean types

Topic groups	See also

A Boolean type is stored as a Byte, a ByteBool is stored as a Byte, a WordBool type is stored as a Word, and a LongBool is stored as a Longint.

A Boolean can assume the values 0 (False) and 1 (True). ByteBool, WordBool, and LongBool types can assume the values 0 (False) or nonzero (True).


Enumerated types
  Object Pascal Language Guide

Enumerated types

Topic groups	See also

An enumerated type is stored as an unsigned byte if the enumeration has no more than 256 values and the type was declared in the {$Z1} state (the default). If an enumerated type has more than 256 values, or if the type was declared in the {$Z2} state, it is stored as an unsigned word. If an enumerated type is declared in the {$Z4} state, it is stored as an unsigned double-word.


Real types
  Object Pascal Language Guide

Real types

Topic groups	See also

The real types store the binary representation of a sign (+ or –), an exponent, and a significand. A real value has the form

+/– significand * 2^exponent

where the significand has a single bit to the left of the binary decimal point. (That is, 0 <= significand < 2.)

In the figures that follow, the most significant bit is always on the left and the least significant bit on the right. The numbers at the top indicate the width (in bits) of each field, with the leftmost items stored at the highest addresses. For example, for a Real48 value, e is stored in the first byte, f in the following five bytes, and s in the most significant bit of the last byte.

The Real48 type

A 6-byte (48-bit) Real48 number is divided into three fields:

If 0 < e <= 255, the value v of the number is given by

v = (–1)^s * 2^(e–129) * (1.f )

If e = 0, then v = 0.

The Real48 type can’t store denormals, NaNs, and infinities. Denormals become zero when stored in a Real48, while NaNs and infinities produce an overflow error if an attempt is made to store them in a Real48.

The Single type

A 4-byte (32-bit) Single number is divided into three fields

The value v of the number is given by

if 0 < e < 255, then v = (–1)^s * 2^(e–127) * (1.f )

if e = 0 and f <> 0, then v = (–1)^s * 2^(–126) * (0.f )

if e = 0 and f = 0, then v = (–1)^s * 0

if e = 255 and f = 0, then v = (–1)^s * Inf

if e = 255 and f <> 0, then v is a NaN

The Double type

An 8-byte (64-bit) Double number is divided into three fields

The value v of the number is given by

if 0 < e < 2047, then v = (–1)^s * 2^(e–1023) * (1.f )

if e = 0 and f <> 0, then v = (–1)^s * 2^(–1022) * (0.f )

if e = 0 and f = 0, then v = (–1)^s * 0

if e = 2047 and f = 0, then v = (–1)^s * Inf

if e = 2047 and f <> 0, then v is a NaN

The Extended type

A 10-byte (80-bit) Extended number is divided into four fields:

The value v of the number is given by

if 0 <= e < 32767, then v = (–1)^s * 2^(e–16383) * (i.f )

if e = 32767 and f = 0, then v = (–1)^s * Inf

if e = 32767 and f <> 0, then v is a NaN

The Comp type

An 8-byte (64-bit) Comp number is stored as a signed 64-bit integer.

The Currency type

An 8-byte (64-bit) Currency number is stored as a scaled and signed 64-bit integer with the four least-significant digits implicitly representing four decimal places.


Pointer types
  Object Pascal Language Guide

Pointer types

Topic groups	See also

A Pointer type is stored in 4 bytes as a 32-bit address. The pointer value nil is stored as zero.


Short string types
  Object Pascal Language Guide

Short string types

Topic groups	See also

A string occupies as many bytes as its maximum length plus one. The first byte contains the current dynamic length of the string, and the following bytes contain the characters of the string.

The length byte and the characters are considered unsigned values. Maximum string length is 255 characters plus a length byte (string[255]).


Long string types
  Object Pascal Language Guide

Long string types

Topic groups	See also

A long string variable occupies four bytes of memory which contain a pointer to a dynamically allocated string. When a long string variable is empty (contains a zero-length string), the string pointer is nil and no dynamic memory is associated with the string variable. For a nonempty string value, the string pointer points to a dynamically allocated block of memory that contains the string value in addition to a 32-bit length indicator and a 32-bit reference count. The table below shows the layout of a long-string memory block.

Offset	Contents
–8	32-bit reference-count
–4	length in bytes
0..Length – 1	character string
Length	NULL character
The NULL character at the end of a long string memory block is automatically maintained by the compiler and the built-in string handling routines. This makes it possible to typecast a long string directly to a null-terminated string.

For string constants and literals, the compiler generates a memory block with the same layout as a dynamically allocated string, but with a reference count of –1. When a long string variable is assigned a string constant, the string pointer is assigned the address of the memory block generated for the string constant. The built-in string handling routines know not to attempt to modify blocks that have a reference count of –1.


Wide string types
  Object Pascal Language Guide

Wide string types

Topic groups	See also

On Windows, a wide string variable occupies four bytes of memory which contain a pointer to a dynamically allocated string. When a wide string variable is empty (contains a zero-length string), the string pointer is nil and no dynamic memory is associated with the string variable. For a nonempty string value, the string pointer points to a dynamically allocated block of memory that contains the string value in addition to a 32-bit length indicator. The table below shows the layout of a wide-string memory block on Windows.

Offset	Contents
–4	32-bit length indicator (in bytes)
0..Length – 1	character string
Length	NULL character
The string length is the number of bytes, so it is twice the number of wide characters contained in the string.

The NULL character at the end of a wide string memory block is automatically maintained by the compiler and the built-in string handling routines. This makes it possible to typecast a wide string directly to a null-terminated string.


Set types
  Object Pascal Language Guide

Set types

Topic groups	See also

A set is a bit array where each bit indicates whether an element is in the set or not. The maximum number of elements in a set is 256, so a set never occupies more than 32 bytes. The number of bytes occupied by a particular set is equal to

(Max div 8) – (Min div 8) + 1

where Max and Min are the upper and lower bounds of the base type of the set. The byte number of a specific element E is

(E div 8) – (Min div 8)

and the bit number within that byte is

E mod 8

where E denotes the ordinal value of the element. When possible, the compiler stores sets in CPU registers, but a set always resides in memory if it is larger than the generic Integer type or if the program contains code that takes the address of the set.


Static array types
  Object Pascal Language Guide

Static array types

Topic groups	See also

A static array is stored as a contiguous sequence of variables of the component type of the array. The components with the lowest indexes are stored at the lowest memory addresses. A multidimensional array is stored with the rightmost dimension increasing first.


Dynamic array types
  Object Pascal Language Guide

Dynamic array types

Topic groups	See also

A dynamic-array variable occupies four bytes of memory which contain a pointer to the dynamically allocated array. When the variable is empty (uninitialized) or holds a zero-length array, the pointer is nil and no dynamic memory is associated with the variable. For a nonempty array, the variable points to a dynamically allocated block of memory that contains the array in addition to a 32-bit length indicator and a 32-bit reference count. The table below shows the layout of a dynamic-array memory block.

Offset	Contents
–8	32-bit reference-count
–4	32-bit length indicator (number of elements)
0..Length * (size of element) – 1	array elements


Record types
  Object Pascal Language Guide

Record types

Topic groups	See also

When a record type is declared in the {$A+} state (the default), and when the declaration does not include a packed modifier, the type is an unpacked record type, and the fields of the record are aligned for efficient access by the CPU. The alignment is controlled by the type of each field. Every data type has an inherent alignment, which is automatically computed by the compiler. The alignment can be 1, 2, 4, or 8, and represents the byte boundary that a value of the type must be stored on to provide the most efficient access. The table below lists the alignments for all data types.

Type	Alignment
Ordinal types	size of the type (1, 2, 4, or 8)
Real types	2 for Real48, 4 for Single, 8 for Double and Extended
Short string types	1
Array types	same as the element type of the array.
Record types	the largest alignment of the fields in the record
Set types	size of the type if 1, 2, or 4, otherwise 1
All other types	4
To ensure proper alignment of the fields in an unpacked record type, the compiler inserts an unused byte before fields with an alignment of 2, and up to three unused bytes before fields with an alignment of 4, if required. Finally, the compiler rounds the total size of the record upward to the byte boundary specified by the largest alignment of any of the fields.

When a record type is declared in the {$A–} state, or when the declaration includes the packed modifier, the fields of the record are not aligned, but are instead assigned consecutive offsets. The total size of such a packed record is simply the size of all the fields. Because data alignment can change, it's a good idea to pack any record structure that you intend to write to disk or pass in memory to another module compiled using a different version of the compiler.


File types
  Object Pascal Language Guide

File types

Topic groups	See also

File types are represented as records. Typed files and untyped files occupy 332 bytes, which are laid out as follows:

type

  TFileRec = packed record

    Handle: Integer;

    Mode: word;

    Flags: word;

    case Byte of

      0: (RecSize: Cardinal);

      1: (BufSize: Cardinal;

          BufPos: Cardinal;

          BufEnd: Cardinal;

          BufPtr: PChar;

          OpenFunc: Pointer;

          InOutFunc: Pointer;

          FlushFunc: Pointer;

          CloseFunc: Pointer;

          UserData: array[1..32] of Byte;

          Name: array[0..259] of Char; );

  end;

Text files occupy 460 bytes, which are laid out as follows:

type

  TTextBuf = array[0..127] of Char;

  TTextRec = packed record

    Handle: Integer;

    Mode: word;

    Flags: word;

    BufSize: Cardinal;

    BufPos: Cardinal;

    BufEnd: Cardinal;

    BufPtr: PChar;

    OpenFunc: Pointer;

    InOutFunc: Pointer;

    FlushFunc: Pointer;

    CloseFunc: Pointer;

    UserData: array[1..32] of Byte;

    Name: array[0..259] of Char;

    Buffer: TTextBuf;

  end;

Handle contains the file’s handle (when the file is open).

The Mode field can assume one of the values

const

  fmClosed = $D7B0;

  fmInput  = $D7B1;

  fmOutput = $D7B2;

  fmInOut  = $D7B3;

where fmClosed indicates that the file is closed, fmInput and fmOutput indicate a text file that has been reset (fmInput) or rewritten (fmOutput), fmInOut indicates a typed or untyped file that has been reset or rewritten. Any other value indicates that the file variable is not assigned (and hence not initialized).

The UserData field is available for user-written routines to store data in.

Name contains the file name, which is a sequence of characters terminated by a null character (#0).

For typed files and untyped files, RecSize contains the record length in bytes, and the Private field is unused but reserved.

For text files, BufPtr is a pointer to a buffer of BufSize bytes, BufPos is the index of the next character in the buffer to read or write, and BufEnd is a count of valid characters in the buffer. OpenFunc, InOutFunc, FlushFunc, and CloseFunc are pointers to the I/O routines that control the file; see Device functions. Flags determines the line break style as follows:

bit 0 clear	LF line breaks
bit 0 set	CRLF line breaks
All other Flags bits are reserved for future use. See also DefaultTextLineBreakStyle and SetLineBreakStyle.


Procedural types
  Object Pascal Language Guide

Procedural types

Topic groups	See also

A procedure pointer is stored as a 32-bit pointer to the entry point of a procedure or function. A method pointer is stored as a 32-bit pointer to the entry point of a method, followed by a 32-bit pointer to an object.


Class types
  Object Pascal Language Guide

Class types

Topic groups	See also

A class-type value is stored as a 32-bit pointer to an instance of the class, which is called an object. The internal data format of an object resembles that of a record. The object’s fields are stored in order of declaration as a sequence of contiguous variables. Fields are always aligned, corresponding to an unpacked record type. Any fields inherited from an ancestor class are stored before the new fields defined in the descendant class.

The first 4-byte field of every object is a pointer to the virtual method table (VMT) of the class. There is exactly one VMT per class (not one per object); distinct class types, no matter how similar, never share a VMT. VMTs are built automatically by the compiler, and are never directly manipulated by a program. Pointers to VMTs, which are automatically stored by constructor methods in the objects they create, are also never directly manipulated by a program.

The layout of a VMT is shown in the following table. At positive offsets, a VMT consists of a list of 32-bit method pointers—one per user-defined virtual method in the class type—in order of declaration. Each slot contains the address of the corresponding virtual method’s entry point. This layout is compatible with a C++ v-table and with COM. At negative offsets, a VMT contains a number of fields that are internal to Object Pascal’s implementation. Applications should use the methods defined in TObject to query this information, since the layout is likely to change in future implementations of Object Pascal.

Offset	Type	Description
–76	Pointer	pointer to virtual method table (or nil)
–72	Pointer	pointer to interface table (or nil)
–68	Pointer	pointer to Automation information table (or nil)
–64	Pointer	pointer to instance initialization table (or nil)
–60	Pointer	pointer to type information table (or nil)
–56	Pointer	pointer to field definition table (or nil)
–52	Pointer	pointer to method definition table (or nil)
–48	Pointer	pointer to dynamic method table (or nil)
–44	Pointer	pointer to short string containing class name
–40	Cardinal	instance size in bytes
–36	Pointer	pointer to a pointer to ancestor class (or nil)
–32	Pointer	pointer to entry point of SafecallException method (or nil)
–28	Pointer	entry point of AfterConstruction method
–24	Pointer	entry point of BeforeDestruction method
–20	Pointer	entry point of Dispatch method
–16	Pointer	entry point of DefaultHandler method
–12	Pointer	entry point of NewInstance method
–8	Pointer	entry point of FreeInstance method
–4	Pointer	entry point of Destroy destructor
0	Pointer	entry point of first user-defined virtual method
4	Pointer	entry point of second user-defined virtual method
...	...	...


Class reference types
  Object Pascal Language Guide

Class reference types

Topic groups	See also

A class-reference value is stored as a 32-bit pointer to the virtual method table (VMT) of a class.


Variant types
  Object Pascal Language Guide

Variant types

Topic groups	See also

A variant is stored as a 16-byte record that contains a type code and a value (or a reference to a value) of the type given by the code. The System and Variants units define constants and types for variants.

The TVarData type represents the internal structure of a Variant variable (on Windows, this is identical to the Variant type used by COM and the Win32 API). The TVarData type can be used in typecasts of Variant variables to access the internal structure of a variable.

The VType field of a TVarData record contains the type code of the variant in the lower twelve bits (the bits defined by the varTypeMask constant). In addition, the varArray bit may be set to indicate that the variant is an array, and the varByRef bit may be set to indicate that the variant contains a reference as opposed to a value.

The Reserved1, Reserved2, and Reserved3 fields of a TVarData record are unused.

The contents of the remaining eight bytes of a TVarData record depend on the VType field. If neither the varArray nor the varByRef bits are set, the variant contains a value of the given type.

If the varArray bit is set, the variant contains a pointer to a TVarArray structure that defines an array. The type of each array element is given by the varTypeMask bits in the VType field.

If the varByRef bit is set, the variant contains a reference to a value of the type given by the varTypeMask and varArray bits in the VType field.

The varString type code is private. Variants containing a varString value should never be passed to a non-Delphi function. On Windows, Delphi's Automation support automatically converts varString variants to varOleStr variants before passing them as parameters to external functions.

On Linux, VT_decimal is not supported.


Program control: Overview
  Object Pascal Language Guide

Program control

Topic groups	See also

These sections explain how parameters and function results are stored and transferred. The final section discusses exit procedures.

Parameters and function results

Parameter passing

Function results

Method calls

Exit procedures


Parameters and function results: Overview
  Object Pascal Language Guide

Parameters and function results

Topic groups	See also

Treatment of parameters and function results is determined by several factors, including calling conventions, parameter semantics, and the type and size of the value being passed.

Parameter passing

Function results

Method calls


Parameter passing
  Object Pascal Language Guide

Parameter passing

Topic groups	See also

Parameters are transferred to procedures and functions via CPU registers or the stack, depending on the routine’s calling convention. For information about calling conventions, see Calling conventions.

Variable (var) parameters are always passed by reference, as 32-bit pointers that point to the actual storage location.

Value and constant (const) parameters are passed by value or by reference, depending on the type and size of the parameter:

An ordinal parameter is passed as an 8-bit, 16-bit, 32-bit, or 64-bit value, using the same format as a variable of the corresponding type.

A real parameter is always passed on the stack. A Single parameter occupies 4 bytes, and a Double, Comp, or Currency parameter occupies 8 bytes. A Real48 occupies 8 bytes, with the Real48 value stored in the lower 6 bytes. An Extended occupies 12 bytes, with the Extended value stored in the lower 10 bytes.

A short-string parameter is passed as a 32-bit pointer to a short string.

A long-string or dynamic-array parameter is passed as a 32-bit pointer to the dynamic memory block allocated for the long string. The value nil is passed for an empty long string.

A pointer, class, class-reference, or procedure-pointer parameter is passed as a 32-bit pointer.

A method pointer is passed on the stack as two 32-bit pointers. The instance pointer is pushed before the method pointer so that the method pointer occupies the lowest address.

Under the register and pascal conventions, a variant parameter is passed as a 32-bit pointer to a Variant value.

Sets, records, and static arrays of 1, 2, or 4 bytes are passed as 8-bit, 16-bit, and 32-bit values. Larger sets, records, and static arrays are passed as 32-bit pointers to the value. An exception to this rule is that records are always passed directly on the stack under the cdecl, stdcall, and safecall conventions; the size of a record passed this way is rounded upward to the nearest double-word boundary.

An open-array parameter is passed as two 32-bit values. The first value is a pointer to the array data, and the second value is one less than the number of elements in the array.

When two parameters are passed on the stack, each parameter occupies a multiple of 4 bytes (a whole number of double words). For an 8-bit or 16-bit parameter, even though the parameter occupies only a byte or a word, it is passed as a double word. The contents of the unused parts of the double word are undefined.

Under the pascal, cdecl, stdcall and safecall conventions, all parameters are passed on the stack. Under the pascal convention, parameters are pushed in the order of their declaration (left-to-right), so that the first parameter ends up at the highest address and the last parameter ends up at the lowest address. Under the cdecl, stdcall, and safecall conventions, parameters are pushed in reverse order of declaration (right-to-left), so that the first parameter ends up at the lowest address and the last parameter ends up at the highest address.

Under the register convention, up to three parameters are passed in CPU registers, and the rest (if any) are passed on the stack. The parameters are passed in order of declaration (as with the pascal convention), and the first three parameters that qualify are passed in the EAX, EDX, and ECX registers, in that order. Real, method-pointer, variant, Int64, and structured types do not qualify as register parameters, but all other parameters do. If more than three parameters qualify as register parameters, the first three are passed in EAX, EDX, and ECX, and the remaining parameters are pushed onto the stack in order of declaration. For example, given the declaration

procedure Test(A: Integer; var B: Char; C: Double; const D: string; E: Pointer);

a call to Test passes A in EAX as a 32-bit integer, B in EDX as a pointer to a Char, and D in ECX as a pointer to a long-string memory block; C and E are pushed onto the stack as two double-words and a 32-bit pointer, in that order.

Register saving conventions

Procedures and functions must preserve the EBX, ESI, EDI, and EBP registers, but can modify the EAX, EDX, and ECX registers. When implementing a constructor or destructor in assembler, be sure to preserve the DL register. Procedures and functions are invoked with the assumption that the CPU’s direction flag is cleared (corresponding to a CLD instruction) and must return with the direction flag cleared.


Function results
  Object Pascal Language Guide

Function results

Topic groups	See also

The following conventions are used for returning function result values.

Ordinal results are returned, when possible, in a CPU register. Bytes are returned in AL, words are returned in AX, and double-words are returned in EAX.

Real results are returned in the floating-point coprocessor’s top-of-stack register (ST(0)). For function results of type Currency, the value in ST(0) is scaled by 10000. For example, the Currency value 1.234 is returned in ST(0) as 12340.

For a string, dynamic array, method pointer, variant, or Int64 result, the effects are the same as if the function result were declared as an additional var parameter following the declared parameters. In other words, the caller passes an additional 32-bit pointer that points to a variable in which to return the function result.

Pointer, class, class-reference, and procedure-pointer results are returned in EAX.

For static-array, record, and set results, if the value occupies one byte it is returned in AL; if the value occupies two bytes it is returned in AX; and if the value occupies four bytes it is returned in EAX. Otherwise, the result is returned in an additional var parameter that is passed to the function after the declared parameters.


Method calls
  Object Pascal Language Guide

Method calls

Topic groups	See also

Methods use the same calling conventions as ordinary procedures and functions, except that every method has an additional implicit parameter Self, which is a reference to the instance or class in which the method is called. The Self parameter is passed as a 32-bit pointer.

Under the register convention, Self behaves as if it were declared before all other parameters. It is therefore always passed in the EAX register.

Under the pascal convention, Self behaves as if it were declared after all other parameters (including the additional var parameter sometimes passed for a function result). It is therefore pushed last, ending up at a lower address than all other parameters.

Under the cdecl, stdcall, and safecall conventions, Self behaves as if it were declared before all other parameters, but after the additional var parameter (if any) passed for a function result. It is therefore the last to be pushed, except for the additional var parameter.

Constructors and destructors


Constructors and destructors
  Object Pascal Language Guide

Constructors and destructors

Topic groups	See also

Constructors and destructors use the same calling conventions as other methods, except that an additional Boolean flag parameter is passed to indicate the context of the constructor or destructor call.

A value of False in the flag parameter of a constructor call indicates that the constructor was invoked through an instance object or using the inherited keyword. In this case, the constructor behaves like an ordinary method. A value of True in the flag parameter of a constructor call indicates that the constructor was invoked through a class reference. In this case, the constructor creates an instance of the class given by Self, and returns a reference to the newly created object in EAX.

A value of False in the flag parameter of a destructor call indicates that the destructor was invoked using the inherited keyword. In this case, the destructor behaves like an ordinary method. A value of True in the flag parameter of a destructor call indicates that the destructor was invoked through an instance object. In this case, the destructor deallocates the instance given by Self just before returning.

The flag parameter behaves as if it were declared before all other parameters. Under the register convention, it is passed in the DL register. Under the pascal convention, it is pushed before all other parameters. Under the cdecl, stdcall, and safecall conventions, it is pushed just before the Self parameter.

Since the DL register indicates whether the constructor or destructor is the outermost in the call stack, you must restore the value of DL before exiting so that BeforeDestruction or AfterConstruction can be called properly.


Exit procedures
  Object Pascal Language Guide

Exit procedures

Topic groups	See also

Exit procedures ensure that specific actions—such as updating and closing files—are carried out before a program terminates. The ExitProc pointer variable allows you to “install” an exit procedure, so that it is always called as part of the program’s termination—whether the termination is normal, forced by a call to Halt, or the result of a runtime error. An exit procedure takes no parameters.

Note:	It is recommended that finalization sections, rather than exit procedures, be used for all exit behavior. Exit procedures are available only for executables, shared objects (Linux) or .DLL (Windows) targets; for packages, exit behavior must be implemented in a finalization section. All exit procedures are called before execution of finalization sections.

Units as well as programs can install exit procedures. A unit can install an exit procedure as part of its initialization code, relying on the procedure to close files or perform other clean-up tasks.

When implemented properly, an exit procedure is part of a chain of exit procedures. The procedures are executed in reverse order of installation, ensuring that the exit code of one unit isn’t executed before the exit code of any units that depend on it. To keep the chain intact, you must save the current contents of ExitProc before pointing it to the address of your own exit procedure. Also, the first statement in your exit procedure must reinstall the saved value of ExitProc.

The following code shows a skeleton implementation of an exit procedure.

var

  ExitSave: Pointer;

procedure MyExit;

begin

  ExitProc := ExitSave;  // always restore old vector first

  ...

end;

begin

  ExitSave := ExitProc;

  ExitProc := @MyExit;

  ...

end.

On entry, the code saves the contents of ExitProc in ExitSave, then installs the MyExit procedure. When called as part of the termination process, the first thing MyExit does is reinstall the previous exit procedure.

The termination routine in the runtime library keeps calling exit procedures until ExitProc becomes nil. To avoid infinite loops, ExitProc is set to nil before every call, so the next exit procedure is called only if the current exit procedure assigns an address to ExitProc. If an error occurs in an exit procedure, it is not called again.

An exit procedure can learn the cause of termination by examining the ExitCode integer variable and the ErrorAddr pointer variable. In case of normal termination, ExitCode is zero and ErrorAddr is nil. In case of termination through a call to Halt, ExitCode contains the value passed to Halt and ErrorAddr is nil. In case of termination due to a runtime error, ExitCode contains the error code and ErrorAddr contains the address of the invalid statement.

The last exit procedure (the one installed by the runtime library) closes the Input and Output files. If ErrorAddr is not nil, it outputs a runtime error message. To output your own runtime error message, install an exit procedure that examines ErrorAddr and outputs a message if it’s not nil; before returning, set ErrorAddr to nil so that the error is not reported again by other exit procedures.

Once the runtime library has called all exit procedures, it returns to the operating system, passing the value stored in ExitCode as a return code.


Inline assembler code: Overview
  Object Pascal Language Guide

Inline assembler code

Topic groups	See also

The built-in assembler allows you to write assembler code within Object Pascal programs. It has the following features:

Allows for inline assembly

Supports all instructions found in the Intel Pentium III, SIMD, and the AMD Athlon (including 3D Now!)

Provides no macro support, but allows for pure assembler function procedures

Permits the use of Object Pascal identifiers, such as constants, types, and variables in assembler statements

As an alternative to the built-in assembler, you can link to object files that contain external procedures and functions. See External declarations for more information.

Note:	If you have external assembler code that you want to use in your applications, you should consider rewriting it in Object Pascal or minimally reimplement it using the inline assembler.

The asm statement

Assembler statement syntax

Expressions

Assembler procedures and functions


The asm statement
  Object Pascal Language Guide

The asm statement

Topic groups	See also

The built-in assembler is accessed through asm statements, which have the form

asm statementList end

where statementList is a sequence of assembler statements separated by semicolons, end-of-line characters, or Object Pascal comments.

Comments in an asm statement must be in Object Pascal style. A semicolon does not indicate that the rest of the line is a comment.

The reserved word inline and the directive assembler are maintained for backward compatibility only. They have no effect on the compiler.

Register use

In general, the rules of register use in an asm statement are the same as those of an external procedure or function. An asm statement must preserve the EDI, ESI, ESP, EBP, and EBX registers, but can freely modify the EAX, ECX, and EDX registers. On entry to an asm statement, BP points to the current stack frame, SP points to the top of the stack, SS contains the segment address of the stack segment, and DS contains the segment address of the data segment. Except for ESP and EBP, an asm statement can assume nothing about register contents on entry to the statement.


Assembler statement syntax
  Object Pascal Language Guide

Assembler statement syntax

Topic groups	See also

This syntax of an assembler statement is

Label: Prefix Opcode Operand1, Operand2

where Label is a label, Prefix is an assembler prefix opcode (operation code), Opcode is an assembler instruction opcode or directive, and Operand is an assembler expression. Label and Prefix are optional. Some opcodes take only one operand, and some take none.

Comments are allowed between assembler statements, but not within them. For example,

MOV AX,1 {Initial value}     { OK }

MOV CX,100 {Count}           { OK }

MOV {Initial value} AX,1;    { Error! }

MOV CX, {Count} 100          { Error! }

Labels

Instruction opcodes

Assembler directives

Operands


Labels
  Object Pascal Language Guide

Labels

Topic groups	See also

Labels are used in built-in assembler statements as they are in Object Pascal—by writing the label and a colon before a statement. There is no limit to a label’s length. As in Object Pascal, labels must be declared in a label declaration part in the block containing the asm statement. There is one exception to this rule: local labels.

Local labels are labels that start with an at-sign (@). They consist of an at-sign followed by one or more letters, digits, underscores, or at-signs. Use of local labels is restricted to asm statements, and the scope of a local label extends from the asm reserved word to the end of the asm statement that contains it. A local label doesn’t have to be declared.


Instruction opcodes
  Object Pascal Language Guide

Instruction opcodes

Topic groups	See also

The built-in assembler supports all of the Intel-documented opcodes for general application use. Note that operating system privileged instructions may not be supported. Specifically, the following families of instructions are supported:

Pentium family

Pentium Pro and Pentium II

Pentium III

Pentium IV

In addition, the built-in assembler supports the following instruction sets

AMD 3DNow! (from the AMD K6 onwards)

AMD Enhanced 3DNow (from the AMD Athlon onwards)

For a complete description of each instruction, refer to your microprocessor documentation.

RET instruction sizing

The RET instruction opcode always generates a near return.

Automatic jump sizing

Unless otherwise directed, the built-in assembler optimizes jump instructions by automatically selecting the shortest, and therefore most efficient, form of a jump instruction. This automatic jump sizing applies to the unconditional jump instruction (JMP), and to all conditional jump instructions when the target is a label (not a procedure or function).

For an unconditional jump instruction (JMP), the built-in assembler generates a short jump (one-byte opcode followed by a one-byte displacement) if the distance to the target label is –128 to 127 bytes. Otherwise it generates a near jump (one-byte opcode followed by a two-byte displacement).

For a conditional jump instruction, a short jump (one-byte opcode followed by a one-byte displacement) is generated if the distance to the target label is –128 to 127 bytes. Otherwise, the built-in assembler generates a short jump with the inverse condition, which jumps over a near jump to the target label (five bytes in total). For example, the assembler statement

JC      Stop

where Stop isn’t within reach of a short jump, is converted to a machine code sequence that corresponds to this:

JNC      Skip

JMP      Stop

Skip:

Jumps to the entry points of procedures and functions are always near.


Assembler directives
  Object Pascal Language Guide

Assembler directives

Topic groups	See also

The built-in assembler supports three assembler define directives: DB (define byte), DW (define word), and DD (define double word). Each generates data corresponding to the comma-separated operands that follow the directive.

The DB directive generates a sequence of bytes. Each operand can be a constant expression with a value between –128 and 255, or a character string of any length. Constant expressions generate one byte of code, and strings generate a sequence of bytes with values corresponding to the ASCII code of each character.

The DW directive generates a sequence of words. Each operand can be a constant expression with a value between –32,768 and 65,535, or an address expression. For an address expression, the built-in assembler generates a near pointer—that is, a word that contains the offset part of the address.

The DD directive generates a sequence of double words. Each operand can be a constant expression with a value between –2,147,483,648 and 4,294,967,295, or an address expression. For an address expression, the built-in assembler generates a far pointer—that is, a word that contains the offset part of the address, followed by a word that contains the segment part of the address.

The DQ directive defines a quadword for Int64 values.

The data generated by the DB, DW, and DD directives is always stored in the code segment, just like the code generated by other built-in assembler statements. To generate uninitialized or initialized data in the data segment, you should use Object Pascal var or const declarations.

Some examples of DB, DW, and DD directives follow.

asm

  DB      0FFH                         { One byte }

  DB      0,99                         { Two bytes }

  DB      'A'                          { Ord('A') }

  DB      'Hello world...',0DH,0AH     { String followed by CR/LF }

  DB      12,"string"                  { Object Pascal style string }

  DW      0FFFFH                       { One word }

  DW      0,9999                       { Two words }

  DW      'A'                          { Same as DB 'A',0 }

  DW      'BA'                         { Same as DB 'A','B' }

  DW      MyVar                        { Offset of MyVar }

  DW      MyProc                       { Offset of MyProc }

  DD      0FFFFFFFFH                   { One double-word }

  DD      0,999999999                  { Two double-words }

  DD      'A'                          { Same as DB 'A',0,0,0 }

  DD      'DCBA'                       { Same as DB 'A','B','C','D' }

  DD      MyVar                        { Pointer to MyVar }

  DD      MyProc                       { Pointer to MyProc }

end;

In Turbo Assembler, when an identifier precedes a DB, DW, or DD directive, it causes the declaration of a byte-, word-, or double-word-sized variable at the location of the directive. For example, Turbo Assembler allows the following:

ByteVar      DB      ?

WordVar      DW      ?

IntVar       DD      ?

  ...

             MOV     AL,ByteVar

             MOV     BX,WordVar

             MOV     ECX,IntVar

The built-in assembler doesn’t support such variable declarations. The only kind of symbol that can be defined in an inline assembler statement is a label. All variables must be declared using Object Pascal syntax; the preceding construction can be replaced by

var

  ByteVar: Byte;

  WordVar: Word;

  IntVar: Integer;

  ...

asm

  MOV     AL,ByteVar

  MOV     BX,WordVar

  MOV     ECX,IntVar

end;

SMALL and LARGE can be used determine the width of a displacement:

MOV eax, [large $1234]

This instruction generates a "normal" move with a 32-bit displacement ($00001234).

MOV eax, [small $1234]

The second instruction will generate a move with an address size override prefix and a 16-bit displacement ($1234).

SMALL can be used to save space. The following example generates an address size override and a 2-byte address (in total three bytes)

MOV eax, [SMALL 123]

as opposed to

mov eax, [123]

which will generate no address size override and a 4-byte address (in total four bytes).

Two additional directives allow assembly code to access dynamic and virtual method: VMTOFFSET and DMTINDEX.

VMTOFFSET retrives the offset in bytes of the virtual method pointer table entry of the virtual method argument from the beginning of the virtual method table (VMT). This directive needs a fully specified class name with a method name as a parameter, for example,TExample.VirtualMethod.

DMTINDEX retrieves the dynamic method table index of the passed dynamic method. This directive also needs a fully specified class name with a method name as a parameter, for example,TExample.DynamicMethod. To invoke the dynamic method, call System.@CallDynaInst with the (E)SI register containing the value obtained from DMTINDEX.

Note:	Methods with the "message" directive, are implemented as dynamic methods and can also be called using the DMTINDEX technique. For example:

    TMyClass = class

      procedure x; message MYMESSAGE;

    end;

The following example uses both DMTINDEX and VMTOFFSET to access dynamic and virtual methods:

program Project2;

type

  TExample = class

    procedure DynamicMethod; dynamic;

    procedure VirtualMethod; virtual;

  end;

procedure TExample.DynamicMethod;

begin

end;

procedure TExample.VirtualMethod;

begin

end;

procedure CallDynamicMethod(e: TExample);

asm

  // Save ESI register

  PUSH    ESI

  // Instance pointer needs to be in EAX

  MOV     EAX, e

  // DMT entry index needs to be in (E)SI

  MOV     ESI, DMTINDEX TExample.DynamicMethod

  // Now call the method

  CALL    System.@CallDynaInst

  // Restore ESI register

  POP ESI

end;

procedure CallVirtualMethod(e: TExample);

asm

  // Instance pointer needs to be in EAX

  MOV     EAX, e

  // Retrieve VMT table entry

  MOV     EDX, [EAX]

  // Now call the method at offset VMTOFFSET

  CALL    DWORD PTR [EDX + VMTOFFSET TExample.VirtualMethod]

end;

var

  e: TExample;

begin

  e := TExample.Create;

  try

    CallDynamicMethod(e);

    CallVirtualMethod(e);

  finally

    e.Free;

  end;

end.


Operands
  Object Pascal Language Guide

Operands

Topic groups	See also

Built-in assembler operands are expressions that consist of constants, registers, symbols, and operators.

Within operands, the following reserved words have predefined meanings

AH	BYTE	DMTINDEX	EDI	HIGH	QWORD	TBYTE
AL	CH	DS	EDX	LARGE	SHL	TYPE
AND	CL	DWORD	EIP	LOW	SHR	VMTOFFSET
AX	CS	DX	ES	MOD	SI	WORD
BH	CX	EAX	ESI	NOT	SMALL	XOR
BL	DH	EBP	ESP	OFFSET	SP
BP	DI	EBX	FS	OR	SS
BX	DL	ECX	GS	PTR	ST
Reserved words always take precedence over user-defined identifiers. For example,

var

  Ch: Char;

  ...

asm

  MOV     CH, 1

end;

loads 1 into the CH register, not into the Ch variable. To access a user-defined symbol with the same name as a reserved word, you must use the ampersand (&) override operator:

MOV     &Ch, 1

It is best to avoid user-defined identifiers with the same names as built-in assembler reserved words.


Expressions: Overview
  Object Pascal Language Guide

Expressions

Topic groups	See also

The built-in assembler evaluates all expressions as 32-bit integer values. It doesn’t support floating-point and string values, except string constants.

Expressions are built from expression elements and operators, and each expression has an associated expression class and expression type.

Differences between Object Pascal and assembler expressions

Expression elements

Expression classes

Expression types

Expression operators


Differences between Object Pascal and assembler expressions
  Object Pascal Language Guide

Differences between Object Pascal and assembler expressions

Topic groups	See also

The most important difference between Object Pascal expressions and built-in assembler expressions is that assembler expressions must resolve to a constant value—a value that can be computed at compile time. For example, given the declarations

const

  X = 10;

  Y = 20;

var

  Z: Integer;

the following is a valid statement.

asm

  MOV     Z,X+Y

end;

Because both X and Y are constants, the expression X + Y is a convenient way of writing the constant 30, and the resulting instruction simply moves of the value 30 into the variable Z. But if X and Y are variables—

var

  X, Y: Integer;

—the built-in assembler cannot compute the value of X + Y at compile time. In this case, to move the sum of X and Y into Z you would use

asm

  MOV     EAX,X

  ADD     EAX,Y

  MOV     Z,EAX

end;

In an Object Pascal expression, a variable reference denotes the contents of the variable. But in an assembler expression, a variable reference denotes the address of the variable. In Object Pascal the expression X + 4 (where X is a variable) means the contents of X plus 4, while to the built-in assembler it means the contents of the word at the address four bytes higher than the address of X. So, even though you’re allowed to write

asm

  MOV     EAX,X+4

end;

this code doesn’t load the value of X plus 4 into AX; instead, it loads the value of a word stored four bytes beyond X. The correct way to add 4 to the contents of X is

asm

  MOV     EAX,X

  ADD     EAX,4

end;


Expression elements: Overview
  Object Pascal Language Guide

Expression elements

Topic groups	See also

The elements of an expression are constants, registers, and symbols.

Constants

Registers

Symbols


Constants
  Object Pascal Language Guide

Constants

Topic groups	See also

The built-in assembler supports two types of constant: numeric constants and string constants.

Numeric constants

Numeric constants must be integers, and their values must be between –2,147,483,648 and 4,294,967,295.

By default, numeric constants use decimal notation, but the built-in assembler also supports binary, octal, and hexadecimal. Binary notation is selected by writing a B after the number, octal notation by writing an O after the number, and hexadecimal notation by writing an H after the number or a $ before the number.

Numeric constants must start with one of the digits 0 through 9 or the $ character. When you write a hexadecimal constant using the H suffix, an extra zero is required in front of the number if the first significant digit is one of the digits A through F. For example, 0BAD4H and $BAD4 are hexadecimal constants, but BAD4H is an identifier because it starts with a letter.

String constants

String constants must be enclosed in single or double quotation marks. Two consecutive quotation marks of the same type as the enclosing quotation marks count as only one character. Here are some examples of string constants:

'Z'

'Delphi'

'Linux'

"That's all folks"

'"That''s all folks," he said.'

'100'

'"'

"'"

String constants of any length are allowed in DB directives, and cause allocation of a sequence of bytes containing the ASCII values of the characters in the string. In all other cases, a string constant can be no longer than four characters and denotes a numeric value which can participate in an expression. The numeric value of a string constant is calculated as

Ord(Ch1) + Ord(Ch2) shl 8 + Ord(Ch3) shl 16 + Ord(Ch4) shl 24

where Ch1 is the rightmost (last) character and Ch4 is the leftmost (first) character. If the string is shorter than four characters, the leftmost characters are assumed to be zero. The following table shows string constants and their numeric values.

String	Value
'a'	00000061H
'ba'	00006261H
'cba'	00636261H
'dcba'	64636261H
'a '	00006120H
'   a'	20202061H
'a' * 2	000000E2H
'a'-'A'	00000020H
not 'a'	FFFFFF9EH


Registers
  Object Pascal Language Guide

Registers

Topic groups	See also

The following reserved symbols denote CPU registers:.

32-bit general purpose	EAX  EBX  ECX  EDX	32-bit pointer or index	ESP  EBP  ESI  EDI
16-bit general purpose	AX BX CX DX	16-bit pointer or index	SP BP SI DI
8-bit low registers	AL BL CL DL	16-bit segment registers	CS DS SS ES
		32-bit segment registers	FS GS
8-bit high registers	AH BH CH DH	Coprocessor register stack	ST
When an operand consists solely of a register name, it is called a register operand. All registers can be used as register operands, and some registers can be used in other contexts.

The base registers (BX and BP) and the index registers (SI and DI) can be written within square brackets to indicate indexing. Valid base/index register combinations are [BX], [BP], [SI], [DI], [BX+SI], [BX+DI], [BP+SI], and [BP+DI]. You can also index with all the 32-bit registers—for example, [EAX+ECX], [ESP], and [ESP+EAX+5].

The segment registers (ES, CS, SS, DS, FS, and GS) are supported, but segments are normally not useful in 32-bit applications.

The symbol ST denotes the topmost register on the 8087 floating-point register stack. Each of the eight floating-point registers can be referred to using ST(X), where X is a constant between 0 and 7 indicating the distance from the top of the register stack.


Symbols
  Object Pascal Language Guide

Symbols

Topic groups	See also

The built-in assembler allows you to access almost all Object Pascal identifiers in assembler expressions, including constants, types, variables, procedures, and functions. In addition, the built-in assembler implements the special symbol @Result, which corresponds to the Result variable within the body of a function. For example, the function

function Sum(X, Y: Integer): Integer;

begin

  Result := X + Y;

end;

could be written in assembler as

function Sum(X, Y: Integer): Integer; stdcall;

begin

  asm

    MOV     EAX,X

    ADD     EAX,Y

    MOV     @Result,EAX

  end;

end;

The following symbols cannot be used in asm statements:

Standard procedures and functions (for example, WriteLn and Chr).

String, floating-point, and set constants (except when loading registers).

Labels that aren’t declared in the current block.

The @Result symbol outside of functions.

The following table summarizes the kinds of symbol that can be used in asm statements.

Symbol	Value	Class	Type
Label	Address of label	Memory reference	Size of type
Constant	Value of constant	Immediate value	0
Type	0	Memory reference	Size of type
Field	Offset of field	Memory	Size of type
Variable	Address of variable	Memory reference	Size of type
Procedure	Address of procedure	Memory reference	Size of type
Function	Address of function	Memory reference	Size of type
Unit	0	Immediate value	0
@Result	Result variable offset	Memory reference	Size of type
With optimizations disabled, local variables (variables declared in procedures and functions) are always allocated on the stack and accessed relative to EBP, and the value of a local variable symbol is its signed offset from EBP. The assembler automatically adds [EBP] in references to local variables. For example, given the declaration

var Count: Integer;

within a function or procedure, the instruction

MOV     EAX,Count

assembles into MOV EAX,[EBP–4].

The built-in assembler treats var parameters as a 32-bit pointers, and the size of a var parameter is always 4. The syntax for accessing a var parameter is different from that for accessing a value parameter. To access the contents of a var parameter, you must first load the 32-bit pointer and then access the location it points to. For example,

function Sum(var X, Y: Integer): Integer; stdcall;

begin

  asm

    MOV     EAX,X

    MOV     EAX,[EAX]

    MOV     EDX,Y

    ADD     EAX,[EDX]

    MOV     @Result,EAX

  end;

end;

Identifiers can be qualified within asm statements. For example, given the declarations

type

  TPoint = record

    X, Y: Integer;

  end;

  TRect = record

    A, B: TPoint;

  end;

var

  P: TPoint;

  R: TRect;

the following constructions can be used in an asm statement to access fields.

MOV     EAX,P.X

MOV     EDX,P.Y

MOV     ECX,R.A.X

MOV     EBX,R.B.Y

A type identifier can be used to construct variables on the fly. Each of the following instructions generates the same machine code, which loads the contents of [EDX] into EAX.

MOV     EAX,(TRect PTR [EDX]).B.X

MOV     EAX,TRect([EDX]).B.X

MOV     EAX,TRect[EDX].B.X

MOV     EAX,[EDX].TRect.B.X


Expression classes
  Object Pascal Language Guide

Expression classes

Topic groups	See also

The built-in assembler divides expressions into three classes: registers, memory references, and immediate values.

An expression that consists solely of a register name is a register expression. Examples of register expressions are AX, CL, DI, and ES. Used as operands, register expressions direct the assembler to generate instructions that operate on the CPU registers.

Expressions that denote memory locations are memory references. Object Pascal’s labels, variables, typed constants, procedures, and functions belong to this category.

Expressions that aren’t registers and aren’t associated with memory locations are immediate values. This group includes Object Pascal’s untyped constants and type identifiers.

Immediate values and memory references cause different code to be generated when used as operands. For example,

const

  Start = 10;

var

  Count: Integer;

  ...

asm

  MOV     EAX,Start            { MOV EAX,xxxx }

  MOV     EBX,Count            { MOV EBX,[xxxx] }

  MOV     ECX,[Start]          { MOV ECX,[xxxx] }

  MOV     EDX,OFFSET Count     { MOV EDX,xxxx }

end;

Because Start is an immediate value, the first MOV is assembled into a move immediate instruction. The second MOV, however, is translated into a move memory instruction, as Count is a memory reference. In the third MOV, the brackets convert Start into a memory reference (in this case, the word at offset 10 in the data segment). In the fourth MOV, the OFFSET operator converts Count into an immediate value (the offset of Count in the data segment).

The brackets and OFFSET operator complement each other. The following asm statement produces identical machine code to the first two lines of the previous asm statement.

asm

  MOV     EAX,OFFSET [Start]

  MOV     EBX,[OFFSET Count]

end;

Memory references and immediate values are further classified as either relocatable or absolute. Relocation is the process by which the linker assigns absolute addresses to symbols. A relocatable expression denotes a value that requires relocation at link time, while an absolute expression denotes a value that requires no such relocation. Typically, expressions that refer to labels, variables, procedures, or functions are relocatable, since the final address of these symbols is unknown at compile time. Expressions that operate solely on constants are absolute.

The built-in assembler allows you to carry out any operation on an absolute value, but it restricts operations on relocatable values to addition and subtraction of constants.


Expression types
  Object Pascal Language Guide

Expression types

Topic groups	See also

Every built-in assembler expression has a type—or, more correctly, a size, because the assembler regards the type of an expression simply as the size of its memory location. For example, the type of an Integer variable is four, because it occupies 4 bytes. The built-in assembler performs type checking whenever possible, so in the instructions

var

  QuitFlag: Boolean;

  OutBufPtr: Word;

  ...

asm

  MOV     AL,QuitFlag

  MOV     BX,OutBufPtr

end;

the assembler checks that the size of QuitFlag is one (a byte), and that the size of OutBufPtr is two (a word). The instruction

MOV     DL,OutBufPtr

produces an error because DL is a byte-sized register and OutBufPtr is a word. The type of a memory reference can be changed through a typecast; these are correct ways of writing the previous instruction:

MOV     DL,BYTE PTR OutBufPtr

MOV     DL,Byte(OutBufPtr)

MOV     DL,OutBufPtr.Byte

These MOV instructions all refer to the first (least significant) byte of the OutBufPtr variable.

In some cases, a memory reference is untyped. One example is an immediate value enclosed in square brackets:

MOV al, [Buffer]

MOV cx, [Buffer]

MOV edx, [Buffer]

The built-in assembler permits both of these instructions, because the expression [100H] has no type—it just means “the contents of address 100H in the data segment,” and the type can be determined from the first operand (byte for AL, word for BX). In cases where the type can’t be determined from another operand, the built-in assembler requires an explicit typecast:

INC     BYTE PTR [ECX]

IMUL    WORD PTR [EDX]

The following table summarizes the predefined type symbols that the built-in assembler provides in addition to any currently declared Object Pascal types.

Symbol	Type
BYTE	1
WORD	2
DWORD	4
QWORD	8
TBYTE	10


Expression operators
  Object Pascal Language Guide

Expression operators

Topic groups	See also

The built-in assembler provides a variety of operators. Precedence rules are different from Object Pascal; for example, in an asm statement, AND has lower precedence than the addition and subtraction operators. The following table lists the built-in assembler’s expression operators in decreasing order of precedence.

Operators	Remarks	Precedence
&		highest
( ), [ ], ., HIGH, LOW
+, –	unary + and –
:
OFFSET, TYPE, PTR, *, /, MOD, SHL, SHR, +, –	binary + and –
NOT, AND, OR, XOR		lowest
The following table defines the built-in assembler’s expression operators.

Operator	Description
&	Identifier override. The identifier immediately following the ampersand is treated as a user-defined symbol, even if the spelling is the same as a built-in assembler reserved symbol.
(...)	Subexpression. Expressions within parentheses are evaluated completely prior to being treated as a single expression element. Another expression can precede the expression within the parentheses; the result in this case is the sum of the values of the two expressions, with the type of the first expression.
[...]	Memory reference. The expression within brackets is evaluated completely prior to being treated as a single expression element. Another expression can precede the expression within the brackets; the result in this case is the sum of the values of the two expressions, with the type of the first expression. The result is always a memory reference.
.	Structure member selector. The result is the sum of the expression before the period and the expression after the period, with the type of the expression after the period. Symbols belonging to the scope identified by the expression before the period can be accessed in the expression after the period.
HIGH	Returns the high-order 8 bits of the word-sized expression following the operator. The expression must be an absolute immediate value.
LOW	Returns the low-order 8 bits of the word-sized expression following the operator. The expression must be an absolute immediate value.
+	Unary plus. Returns the expression following the plus with no changes. The expression must be an absolute immediate value.
–	Unary minus. Returns the negated value of the expression following the minus. The expression must be an absolute immediate value.
+	Addition. The expressions can be immediate values or memory references, but only one of the expressions can be a relocatable value. If one of the expressions is a relocatable value, the result is also a relocatable value. If either of the expressions is a memory reference, the result is also a memory reference.
–	Subtraction. The first expression can have any class, but the second expression must be an absolute immediate value. The result has the same class as the first expression.
:	Segment override. Instructs the assembler that the expression after the colon belongs to the segment given by the segment register name (CS, DS, SS, FS, GS, or ES) before the colon. The result is a memory reference with the value of the expression after the colon. When a segment override is used in an instruction operand, the instruction is prefixed with an appropriate segment-override prefix instruction to ensure that the indicated segment is selected.
OFFSET	Returns the offset part (double word) of the expression following the operator. The result is an immediate value.
TYPE	Returns the type (size in bytes) of the expression following the operator. The type of an immediate value is 0.
PTR	Typecast operator. The result is a memory reference with the value of the expression following the operator and the type of the expression in front of the operator.
*	Multiplication. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
/	Integer division. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
MOD	Remainder after integer division. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
SHL	Logical shift left. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
SHR	Logical shift right. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
NOT	Bitwise negation. The expression must be an absolute immediate value, and the result is an absolute immediate value.
AND	Bitwise AND. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
OR	Bitwise OR. Both expressions must be absolute immediate values, and the result is an absolute immediate value.
XOR	Bitwise exclusive OR. Both expressions must be absolute immediate values, and the result is an absolute immediate value.


Assembler procedures and functions
  Object Pascal Language Guide

Assembler procedures and functions

Topic groups	See also

You can write complete procedures and functions using inline assembler code, without including a begin...end statement. For example,

function LongMul(X, Y: Integer): Longint;

asm

  MOV     EAX,X

  IMUL    Y

end;

The compiler performs several optimizations on these routines:

No code is generated to copy value parameters into local variables. This affects all string-type value parameters and other value parameters whose size isn’t 1, 2, or 4 bytes. Within the routine, such parameters must be treated as if they were var parameters.

Unless a function returns a string, variant, or interface reference, the compiler doesn’t allocate a function result variable; a reference to the @Result symbol is an error. For strings, variants, and interfaces, the caller always allocates an @Result pointer.

The compiler only generates stack frames for nested routines, for routines that have local parameters, or for routines that have parameters on the stack.

The automatically generated entry and exit code for the routine looks like this:

PUSH    EBP              ;Present if Locals <> 0 or Params <> 0

MOV     EBP,ESP          ;Present if Locals <> 0 or Params <> 0

SUB     ESP,Locals       ;Present if Locals <> 0

 ...

MOV     ESP,EBP          ;Present if Locals <> 0

POP     EBP              ;Present if Locals <> 0 or Params <> 0

RET     Params           ;Always present

If locals include variants, long strings, or interfaces, they are initialized to zero but not finalized.

Locals is the size of the local variables and Params is the size of the parameters. If both Locals and Params are zero, there is no entry code, and the exit code consists simply of a RET instruction.

Assembler functions return their results as follows.

Ordinal values are returned in AL (8-bit values), AX (16-bit values), or EAX (32-bit values).

Real values are returned in ST(0) on the coprocessor’s register stack. (Currency values are scaled by 10000.)

Pointers, including long strings, are returned in EAX.

Short strings and variants are returned in the temporary location pointed to by @Result.


Formal grammar
  Object Pascal Language Guide

Object Pascal grammar

Topic groups	See also

Goal -> (Program | Package  | Library  | Unit)

Program -> [PROGRAM Ident ['(' IdentList ')'] ';']

           ProgramBlock '.'

Unit -> UNIT Ident ';'

        InterfaceSection

        ImplementationSection

        InitSection '.'

Package -> PACKAGE Ident ';'

           [RequiresClause]

           [ContainsClause]

           END '.'

Library -> LIBRARY Ident ';'

           ProgramBlock '.'

ProgramBlock -> [UsesClause]

                Block

UsesClause -> USES IdentList ';'

InterfaceSection -> INTERFACE

                    [UsesClause]

                    [InterfaceDecl]...

InterfaceDecl -> ConstSection

              -> TypeSection

              -> VarSection

              -> ExportedHeading

ExportedHeading -> ProcedureHeading ';' [Directive]

                -> FunctionHeading ';' [Directive]

ImplementationSection -> IMPLEMENTATION

                         [UsesClause]

                         [DeclSection]...

Block -> [DeclSection]

         CompoundStmt

DeclSection -> LabelDeclSection

            -> ConstSection

            -> TypeSection

            -> VarSection

            -> ProcedureDeclSection

LabelDeclSection -> LABEL LabelId

ConstSection -> CONST (ConstantDecl ';')...

ConstantDecl -> Ident '=' ConstExpr

             -> Ident ':' TypeId '=' TypedConstant

TypeSection -> TYPE (TypeDecl ';')...

TypeDecl -> Ident '=' Type

         -> Ident '=' RestrictedType

TypedConstant -> (ConstExpr | ArrayConstant | RecordConstant)

ArrayConstant -> '(' TypedConstant/','... ')'

RecordConstant -> '(' RecordFieldConstant/';'... ')'

RecordFieldConstant -> Ident ':' TypedConstant

Type -> TypeId

     -> SimpleType

     -> StrucType

     -> PointerType

     -> StringType

     -> ProcedureType

     -> VariantType

     -> ClassRefType

RestrictedType -> ObjectType

               -> ClassType

               -> InterfaceType

ClassRefType -> CLASS OF TypeId

SimpleType -> (OrdinalType | RealType)

RealType -> REAL48

         -> REAL

         -> SINGLE

         -> DOUBLE

         -> EXTENDED

         -> CURRENCY

         -> COMP

OrdinalType -> (SubrangeType | EnumeratedType | OrdIdent)

OrdIdent -> SHORTINT

         -> SMALLINT

         -> INTEGER

         -> BYTE

         -> LONGINT

         -> INT64

         -> WORD

         -> BOOLEAN

         -> CHAR

         -> WIDECHAR

         -> LONGWORD

         -> PCHAR

VariantType -> VARIANT

            -> OLEVARIANT

SubrangeType -> ConstExpr '..' ConstExpr

EnumeratedType -> '(' EnumeratedTypeElement/','... ')'

EnumeratedTypeElement -> Ident [ '=' ConstExpr ]

StringType -> STRING

           -> ANSISTRING

           -> WIDESTRING

           -> STRING '[' ConstExpr ']'

StrucType -> [PACKED] (ArrayType | SetType | FileType | RecType)

ArrayType -> ARRAY ['[' OrdinalType/','... ']'] OF Type

RecType -> RECORD [FieldList] END

FieldList ->  FieldDecl/';'... [VariantSection] [';']

FieldDecl -> IdentList ':' Type

VariantSection -> CASE [Ident ':'] TypeId OF RecVariant/';'...

RecVariant -> ConstExpr/','...  ':' '(' [FieldList] ')'

SetType -> SET OF OrdinalType

FileType -> FILE OF TypeId

PointerType -> '^' TypeId

ProcedureType -> (ProcedureHeading | FunctionHeading) [OF OBJECT]

VarSection -> VAR (VarDecl ';')...

VarDecl -> IdentList ':' Type [(ABSOLUTE (Ident | ConstExpr)) | '=' ConstExpr]

Expression -> SimpleExpression [RelOp SimpleExpression]...

SimpleExpression -> ['+' | '-'] Term [AddOp Term]...

Term -> Factor [MulOp Factor]...

Factor -> Designator ['(' ExprList ')']

       -> '@' Designator

       -> Number

       -> String

       -> NIL

       -> '(' Expression ')'

       -> NOT Factor

       -> SetConstructor

       -> TypeId '(' Expression ')'

RelOp -> '>'

      -> '<'

      -> '<='

      -> '>='

      -> '<>'

      -> IN

      -> IS

      -> AS

AddOp -> '+'

      -> '-'

      -> OR

      -> XOR

MulOp -> '*'

      -> '/'

      -> DIV

      -> MOD

      -> AND

      -> SHL

      -> SHR

Designator -> QualId ['.' Ident | '[' ExprList ']' | '^']...

SetConstructor -> '[' [SetElement/','...] ']'

SetElement -> Expression ['..' Expression]

ExprList -> Expression/','...

Statement -> [LabelId ':'] [SimpleStatement | StructStmt]

StmtList -> Statement/';'...

SimpleStatement -> Designator ['(' ExprList ')']

                -> Designator ':=' Expression

                -> INHERITED

                -> GOTO LabelId

StructStmt -> CompoundStmt

           -> ConditionalStmt

           -> LoopStmt

           -> WithStmt

CompoundStmt -> BEGIN StmtList END

ConditionalStmt -> IfStmt

                -> CaseStmt

IfStmt -> IF Expression THEN Statement [ELSE Statement]

CaseStmt -> CASE Expression OF CaseSelector/';'... [ELSE StmtList] [';'] END

CaseSelector -> CaseLabel/','... ':' Statement

CaseLabel -> ConstExpr ['..' ConstExpr]

LoopStmt -> RepeatStmt

         -> WhileStmt

         -> ForStmt

RepeatStmt -> REPEAT Statement UNTIL Expression

WhileStmt -> WHILE Expression DO Statement

ForStmt -> FOR QualId ':=' Expression (TO | DOWNTO) Expression DO Statement

WithStmt -> WITH IdentList DO Statement

ProcedureDeclSection -> ProcedureDecl

                     -> FunctionDecl

ProcedureDecl -> ProcedureHeading ';' [Directive]

                 Block ';'

FunctionDecl -> FunctionHeading ';' [Directive]

                Block ';'

FunctionHeading -> FUNCTION Ident [FormalParameters] ':' (SimpleType | STRING)

ProcedureHeading -> PROCEDURE Ident [FormalParameters]

FormalParameters -> '(' FormalParm/';'... ')'

FormalParm -> [VAR | CONST | OUT] Parameter

Parameter -> IdentList  [':' ([ARRAY OF] SimpleType | STRING | FILE)]

          -> Ident ':' SimpleType '=' ConstExpr

Directive -> CDECL

          -> REGISTER

          -> DYNAMIC

          -> VIRTUAL

          -> EXPORT

          -> EXTERNAL

          -> FAR

          -> FORWARD

          -> MESSAGE

          -> OVERRIDE

          -> OVERLOAD

          -> PASCAL

          -> REINTRODUCE

          -> SAFECALL

          -> STDCALL

ObjectType -> OBJECT [ObjHeritage] [ObjFieldList] [MethodList] END

ObjHeritage -> '(' QualId ')'

MethodList -> (MethodHeading [';' VIRTUAL])/';'...

MethodHeading -> ProcedureHeading

              -> FunctionHeading

              -> ConstructorHeading

              -> DestructorHeading

ConstructorHeading -> CONSTRUCTOR Ident [FormalParameters]

DestructorHeading -> DESTRUCTOR Ident [FormalParameters]

ObjFieldList -> (IdentList ':' Type)/';'...

InitSection -> INITIALIZATION StmtList [FINALIZATION StmtList] END

            -> BEGIN StmtList END

            -> END

ClassType -> CLASS [ClassHeritage]

             [ClassFieldList]

             [ClassMethodList]

             [ClassPropertyList]

             END

ClassHeritage -> '(' IdentList ')'

ClassVisibility -> [PUBLIC | PROTECTED | PRIVATE | PUBLISHED]

ClassFieldList -> (ClassVisibility ObjFieldList)/';'...

ClassMethodList -> (ClassVisibility MethodList)/';'...

ClassPropertyList -> (ClassVisibility PropertyList ';')...

PropertyList -> PROPERTY  Ident [PropertyInterface]  PropertySpecifiers

PropertyInterface -> [PropertyParameterList] ':' Ident

PropertyParameterList -> '[' (IdentList ':' TypeId)/';'... ']'

PropertySpecifiers -> [INDEX ConstExpr]

                      [READ Ident]

                      [WRITE Ident]

                      [STORED (Ident | Constant)]

                      [(DEFAULT ConstExpr) | NODEFAULT]

                      [IMPLEMENTS TypeId]

InterfaceType -> INTERFACE [InterfaceHeritage]

                 [ClassMethodList]

                 [ClassPropertyList]

                 END

InterfaceHeritage -> '(' IdentList ')'

RequiresClause -> REQUIRES IdentList... ';'

ContainsClause -> CONTAINS IdentList... ';'

IdentList -> Ident/','...

QualId -> [UnitId '.'] Ident

TypeId -> [UnitId '.'] <type-identifier>

Ident -> <identifier>

ConstExpr -> <constant-expression>

UnitId -> <unit-identifier>

LabelId -> <label-identifier>

Number -> <number>

String -> <string>


Related topic groups
Object Pascal Language Guide

•	Inline assembler code

•	Classes and objects

•	Program control

•	Data types, variables and constants

•	Dynamic-link libraries and packages

•	Object Pascal grammar

•	Memory management

•	Object interfaces

•	Object Pascal overview

•	Procedures and functions

•	Programs and units

•	Standard routines and I/O

•	Syntactic elements


Inline assembler code

Related topic groups

•	Inline assembler code: Overview

•	The asm statement

•	Assembler statement syntax

•	Labels

•	Instruction opcodes

•	Assembler directives

•	Operands

•	Expressions: Overview

•	Differences between Object Pascal and assembler expressions

•	Expression elements: Overview

•	Constants

•	Registers

•	Symbols

•	Expression classes

•	Expression types

•	Expression operators

•	Assembler procedures and functions


Classes and objects

Related topic groups

•	Classes and objects: Overview

•	About class types

•	Inheritance and scope

•	TObject and TClass

•	Compatibility of class types

•	Object types

•	Visibility of class members

•	Private, protected, and public members

•	Published members

•	Automated members

•	Forward declarations and mutually dependent classes

•	Fields

•	Methods: Overview

•	Method declarations and implementations

•	Inherited

•	Self

•	Method binding: Overview

•	Static methods

•	Virtual and dynamic methods

•	Overriding versus hiding

•	Reintroduce

•	Abstract methods

•	Overloading methods

•	Constructors

•	Destructors

•	Message methods

•	Properties: Overview

•	Property access

•	Array properties

•	Index specifiers

•	Storage specifiers

•	Property overrides and redeclarations

•	Class references: Overview

•	Class-reference types

•	Constructors and class references

•	Class operators: Overview

•	The is operator

•	The as operator

•	Class methods

•	Exceptions: Overview

•	When to use exceptions

•	Declaring exception types

•	Raising and handling exceptions

•	Try...except statements

•	Re-raising exceptions

•	Nested exceptions

•	Try...finally statements

•	Standard exception classes and routines


Program control

Related topic groups

•	Program control: Overview

•	Parameters and function results: Overview

•	Parameter passing

•	Function results

•	Method calls

•	Constructors and destructors

•	Exit procedures


Data types, variables and constants

Related topic groups

•	Data types and variables: Overview

•	About types

•	Simple types: Overview

•	Ordinal types: Overview

•	Integer types

•	Character types

•	Boolean types

•	Enumerated types

•	Subrange types

•	Real types

•	About string types

•	Short strings

•	Long strings

•	WideString

•	About extended character sets

•	Working with null-terminated strings

•	Using pointers, arrays, and string constants

•	Mixing Pascal strings and null-terminated strings

•	Structured types: Overview

•	Sets

•	Arrays: Overview

•	Static arrays

•	Dynamic arrays

•	Multidimensional dynamic arrays

•	Array types and assignments

•	About records

•	Variant parts in records

•	File types

•	Pointers and pointer types

•	Overview of pointers

•	About pointer types

•	Character pointers

•	Other standard pointer types

•	Procedural types: Overview

•	Procedural types in statements and expressions

•	Variant types: Overview

•	Variant type conversions

•	Variants in expressions

•	Variant arrays

•	OleVariant

•	Type compatibility and identity: Overview

•	Type identity

•	Type compatibility

•	Assignment-compatibility

•	Declaring types

•	Variables: Overview

•	Declaring variables

•	Absolute addresses

•	Dynamic variables

•	Thread-local variables

•	Constants: Overview

•	True constants

•	Constant expressions

•	Resource strings

•	About typed constants

•	Array constants

•	Record constants

•	Procedural constants

•	Pointer constants


Dynamic-link libraries and packages

Related topic groups

•	Libraries and packages: Overview

•	Calling dynamically loadable libraries

•	Dynamic loading

•	Writing dynamically loadable libraries

•	The exports clause

•	Library initialization code

•	Global variables in a library

•	Libraries and system variables

•	Exceptions and runtime errors in libraries

•	Shared-memory manager (Windows only)

•	Packages: Overview

•	Package declarations and source files

•	Naming packages

•	The requires clause

•	The contains clause

•	Compiling packages: Overview

•	Generated files

•	Package-specific compiler directives

•	Package-specific command-line compiler switches


Object Pascal grammar

Related topic groups

•	Formal grammar


Memory management

Related topic groups

•	Memory management: Overview

•	Variables

•	Integer types

•	Character types

•	Boolean types

•	Enumerated types

•	Real types

•	Pointer types

•	Short string types

•	Long string types

•	Wide string types

•	Set types

•	Static array types

•	Dynamic array types

•	Record types

•	File types

•	Procedural types

•	Class types

•	Class reference types

•	Variant types


Object interfaces

Related topic groups

•	Object interfaces: Overview

•	Interface types: Overview

•	IInterface and inheritance

•	Interface identification

•	Calling conventions for interfaces

•	Interface properties

•	Forward declarations

•	Implementing interfaces

•	Method resolution clauses

•	Changing inherited implementations

•	Implementing interfaces by delegation

•	Delegating to an interface-type property

•	Delegating to a class-type property

•	Interface references

•	Interface assignment-compatibility

•	Interface typecasts

•	Interface querying

•	Automation objects (Windows only)

•	Dispatch interface types (Windows only)

•	Accessing Automation objects (Windows only)

•	Dual interfaces (Windows only)


Object Pascal overview

Related topic groups

•	Using Object Pascal

•	Program organization: Overview

•	Pascal source files

•	Other files used to build applications

•	Compiler-generated files

•	About example programs

•	A simple console application

•	A more complicated example

•	A native application


Procedures and functions

Related topic groups

•	Procedures and functions: Overview

•	Declaring procedures and functions: Overview

•	Procedure declarations

•	Function declarations

•	Calling conventions

•	Forward and interface declarations

•	External declarations

•	Overloading procedures and functions

•	Local declarations

•	Parameters: Overview

•	Parameter semantics: Overview

•	Value and variable parameters

•	Constant parameters

•	Out parameters

•	Untyped parameters

•	About string parameters

•	Array parameters: Overview

•	Open array parameters

•	Variant open array parameters

•	Default parameters

•	Default parameters and overloaded routines

•	Default parameters in forward and interface declarations

•	Calling procedures and functions

•	Open array constructors


Programs and units

Related topic groups

•	Programs and units: Overview

•	Program structure and syntax: Overview

•	The program heading

•	The program uses clause

•	The block

•	Unit structure and syntax: Overview

•	The unit heading

•	The interface section

•	The implementation section

•	The initialization section

•	The finalization section

•	Unit references and the uses clause

•	The syntax of a uses clause

•	Multiple and indirect unit references

•	Circular unit references


Standard routines and I/O

Related topic groups

•	Standard routines and I/O: Overview

•	File input and output

•	Text files

•	Untyped files

•	Text-file device drivers: Overview

•	Device functions

•	Handling null-terminated strings

•	Other standard routines


Syntactic elements

Related topic groups

•	Syntactic elements: Overview

•	Fundamental syntactic elements: Overview

•	Special symbols

•	Identifiers

•	Reserved words

•	Directives

•	Numerals

•	Labels

•	Character strings

•	Comments and compiler directives

•	About expressions

•	About operators

•	Arithmetic operators

•	Boolean operators

•	Logical (bitwise) operators

•	String operators

•	Pointer operators

•	Set operators

•	Relational operators

•	Class operators

•	The @ operator

•	Operator precedence rules

•	Function calls

•	Set constructors

•	Indexes

•	Typecasts: Overview

•	Value typecasts

•	Variable typecasts

•	About declarations and statements

•	Declarations

•	Statements

•	Simple statements: Overview

•	Assignment statements

•	Procedure and function calls

•	Goto statements

•	Structured statements: Overview

•	Compound statements

•	With statements

•	If statements

•	Case statements

•	Control loops

•	Repeat statements

•	While statements

•	For statements

•	Blocks and scope: Overview

•	Blocks

•	Scope


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