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Options that Control C++ Dialect

The following documentation describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program uses. For instance, you might compile a file, firstClass.C, like the following example.

    g++  -g  -felide-constructors  -O  -c  firstClass.C

In the previous example, only -felide-constructors is an option meant only for C++ programs; you can use the other options with any language supported by GNU CC. The following lists options that are only for compiling C++ programs.

-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

-fall-virtual
Treat all possible member functions as virtual, implicitly. All member functions (except for constructor functions and new or delete member operators) are treated as virtual functions of the class where they appear.
This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case.

-fcheck-new
Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. The current ANSI C++ Working Paper requires that operator new never return a null pointer, so this check is normally unnecessary.

-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged.

-fdollars-in-identifiers
Accept the dollar sign ($) in identifiers. You can also explicitly prohibit use of $ with the -fno-dollars-in-identifiers option. GNU C++ allows $ by default on some target systems but not others. Traditional C allowed the character $, to form part of identifiers. However, ANSI C and C++ forbids $ in identifiers.

-fenum-int-equiv
Anachronistically permit implicit conversion of int to enumeration types. Current C++ allows conversion of enum to int, but not the reverse.

-fexternal-templates
Cause template instantiations to obey #pragma interface and implementation; template instances are emitted or not according to the location of the template definition. See Wheres the Template? for more explanation of templates .

-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted or not according to the place where they are first instantiated. See Wheres the Template? for more explanation on templates.

-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the draft C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of gcc, and other (traditional) implementations of C++.

The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-fno-gnu-keywords
Do not recognize classof, headof, signature, sigof or typeof as a keyword, so that code can use these words as identifiers. Instead, use the keywords, __classof__, __ headof__, __signature__, __sigof__, and __typeof__. -ansi implies -fno-gnu-keywords.

-fno-implicit-templates
Never emit code for templates which are instantiated implicitly (i.e., by use); only emit code for explicit instantiations. See Wheres the Template? for more explanation on templates.

-fhandle-signatures
Recognize the signature and sigof keywords for specifying abstract types. The default (-fno-handle-signatures) is not to recognize them. See Type Abstraction using Signatures.

-fhuge-objects
Support virtual function calls for objects that exceed the size representable by a short int. Users should not use this flag by default; if you need to use it, the compiler will tell you so. If you compile any of your code with this flag, you must compile all of your code with this flag (including libg++, if you use it). This flag is not useful when compiling with -fvtable-thunks.

-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This will cause linker errors if these functions are not inlined everywhere they are called.

-fmemoize-lookups
-fsave-memoized
Use heuristics to compile faster. These heuristics are not enabled by default, since they are only effective for certain input files. Other input files compile more slowly.
The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like the following makes six passes through all three steps.

    cout << "This " << p << " has " << n << " legs.\n";

By using a software cache, a hit significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. -fmemoize-lookups enables the software cache.
Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the -fmemoize-lookups flag, the cache is flushed after every function that is compiled. The -fsave-memoized flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed.
The code that implements these flags has rotted; you should probably avoid using them.

-fstrict-prototype
Within an extern C linkage specification, treat a function declaration with no arguments, such as int foo();, as declaring the function to take no arguments. Normally, such a declaration means that the function foo can take any combination of arguments, as in C. -pedantic implies -fstrict-prototype unless overridden with -fno-strict-prototype.
This flag no longer affects declarations with C++ linkage.

-fno-nonnull-objects
Dont assume that a reference is initialized to refer to a valid object. Although the current C++ Working Paper prohibits null references, some old code may rely on them; you can use -fno-nonnull-objects to turn on checking.
At the moment, the compiler only does this checking for conversions to virtual base classes.

-foperator-names
Recognize the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms for the symbols they refer to. -ansi implies -foperator-names.

-frepo
Enable automatic template instantiation. This option also implies -fno-implicit-templates. See Wheres the Template? for more explanation of templates.

-fthis-is-variable
Permit assignment to this. The incorporation of user-defined free store management into C++ has made assignment to this an anachronism. Therefore, by default it is invalid to assign to this within a class member function; that is, GNU C++ treats this in a member function of class X as a non-lvalue of type X*
However, for backwards compatibility, you can make it valid with -fthis-is-variable.

-fvtable-thunks
Use thunks to implement the virtual function dispatch table (vtable). The traditional (cfront-style) approach to implementing vtables was to store a pointer to the function and two offsets for adjusting the this pointer at the call site. Newer implementations store a single pointer to a thunk function which does any necessary adjustment and then calls the target function.
This option also enables a heuristic for controlling emission of vtables; if a class has any non-inline virtual functions, the vtable will be emitted in the translation unit containing the first one of those.

-nostdinc
Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building libg++.)

-traditional
For C++ programs (in addition to the effects that apply to both C and C++), this has the same effect as -fthis-is-variable. See Options Controlling C Dialect.

In addition, the following options for optimization, warning, and code generation have meanings only for C++ programs.

-fno-default-inline
Do not assume inline for functions defined inside a class scope. See Options That Control Optimization.

-Woverloaded-virtual
-Wtemplate-debugging
Warnings that apply only to C++ programs. See Options to Request or Suppress Warnings.

+en
Control how virtual function definitions are used, in a fashion compatible with cfront 1.x. For more explanation of compatibility conventions for this option, see the description for +e in Options for Code Generation Conventions.