Expression defaults for generic formal functions (GNAT Reference Manual)

#### 17.2.5 Expression defaults for generic formal functions ¶

The declaration of a generic formal function is allowed to specify an expression as a default, using the syntax of an expression function.

Here is an example of this feature:

    generic
       type T is private;
       with function Copy (Item : T) return T is (Item); -- Defaults to Item
    package Stacks is

       type Stack is limited private;

       procedure Push (S : in out Stack; X : T); -- Calls Copy on X
       function Pop (S : in out Stack) return T; -- Calls Copy to return item

    private
       -- ...
    end Stacks;

If Stacks is instantiated with an explicit actual for Copy, then that will be called when Copy is called in the generic body. If the default is used (i.e. there is no actual corresponding to Copy), then calls to Copy in the instance will simply return Item.