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From: Brian Rogoff <bpr@shell5.ba.best.com>
Subject: Re: packet type ?
Date: 2000/01/19
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Date: 2000-01-19T00:00:00+00:00
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On 19 Jan 2000, Anders Gidenstam wrote:
> Well, you make a generic package containing the stack ADT.
> It'll be something like this:
> 
> generic
> 	type Element_Type is private;
> package Stack_ADT is
> 
> 	type Stack(Size : Natural) is private;
> 
> 	procedure Push (S : in out Stack; I : in  Element_Type);
> 	procedure Pop  (S : in out Stack; I : out Element_Type);
> 	
> 	...
> 
> private
> 	type Element_Array is array (Positive range <>) of Element_Type;
> 
> 	type Stack(Size : Natural) is
> 		record
> 			S   : Element_Array (1..Size);
> 			Top : Natural := 0;
> 		end record;
> end Stack_ADT;
> 
> And you use it like this:
> 
> with Stack_ADT;
> procedure Something is
> 
> 	package Integer_Stack is new Stack_ADT (Integer);
> 	use 	Integer_Stack;
> 	package Float_Stack is new Stack_ADT (Float);
> 	use 	Float_Stack;
> 
> 
> 	My_Stack : Integer_Stack.Stack (Size => 100);
> 	Some_Stack : Float_Stack.Stack (Size => 50);
> begin
> 	Push (My_Stack, 3);
> 	Push (Some_Stack, 7.0);
> 	...
> 
> I hope this was helpful in understanding this part of Ada.
> (Note that this isn't OO in the C++ sense, you can't let any type inherit 
>  the stack's properties but I don't think that type of inheritance is needed 
>  for an ADT like this. And of course one could make the stack a tagged type
>  which would allow inheritance.)

Ada 95 also allows you to create a stack signature 

generic 
    type Item_Type is private;
    type Stack_Type is private;
    with procedure Push(Stack : in out Stack_Type; Item : in Item_Type);
    with procedure Pop(Stack : in out Stack_Type; Item : out Item_Type);
    ...
package Stack_Sig is begin end;

and write code which only depends on an instantiation having that profile,
so that your stack code and many others can supply the stack ADT for the 
code. This is a very nice feature of Ada 95 which is not described in many 
Ada books; Barnes and Ben-Ari are exceptional here.

-- Brian