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.