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In Out Parameters For Functions (Matthew Heaney)

Although Ada doesn't officially have in out parameters for functions, you can still give a function behavior that amounts to the same thing.

One useful thing to be able to do is to modify the item on top of the stack, without actually copying a new item to the top. This doesn't mean replacing the item on the top with another item that has a different value -- it means modifying the actual item, in place.

What we do is to return a reference to the top item. Ada doesn't have reference types like you have in C++, so we use access types instead, which give you the same thing with only a small amount of additional syntactic overhead.

The operation Set_Top is _function_ that allows the client to modify the item on top of the stack, by returning a pointer to the item on top:

   type Item_Access is
      access all Item_Type;

   function Set_Top
     (Stack : Root_Stack)
     return Item_Access is abstract;

You use it as follows:

   Set_Top (Stack).all := ;

or, if the Item_Type is a record:

   Set_Top (Stack). := ;

or, passing it to a modifier:

   Do_Something (To_The_Object => Set_Top (Stack).all);

For example, suppose a stack of integers has the following value (top to bottom):

3 2 1

If I say

  Set_Top (Stack).all := 4;

then the stack now looks like

4 2 1


(c) 1998-2004 All Rights Reserved David Botton