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| Ruby user's guide | Singleton methods |

The behavior of an instance is determined by its class, but there may be times we know that a particular instance should have special behavior. In most languages, we must go to the trouble of defining another class, which would then only be instantiated once. In ruby we can give any object its own methods.

    ruby> class SingletonTest
        |   def size
        |     print "25\n"
        |   end
        | end
       nil
    ruby> test1 = SingletonTest.new
       #<SingletonTest:0xbc468>
    ruby> test2 = SingletonTest.new
       #<SingletonTest:0xbae20>
    ruby> def test2.size
        |   print "10\n"
        | end
       nil
    ruby> test1.size
    25
       nil
    ruby> test2.size
    10
       nil

In this example, `test1` and `test2` belong to same class, but `test2` has been given a redefined `size` method and so they behave differently. A method given only to a single object is called a singleton method.

Singleton methods are often used for elements of a graphic user interface (GUI), where different actions need to be taken when different buttons are pressed.

Singleton methods are not unique to ruby, as they appear in CLOS, Dylan, etc. Also, some languages, for example, Self and NewtonScript, have singleton methods only. These are sometimes called prototype-based languages.

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