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From: d96andgi@dtek.chalmers.se (Anders Gidenstam)
Subject: Re: packet type ?
Date: 2000/01/19
Message-ID: <8646k7$6g1$1@nyheter.chalmers.se>#1/1
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References: <387F3DD1.485F7C70@icn.siemens.de> <85njs1$3rl$1@nnrp1.deja.com>
 <38845EBD.6F90845D@icn.siemens.de>
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Newsgroups: comp.lang.ada
Date: 2000-01-19T11:19:35+00:00
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In article <38845EBD.6F90845D@icn.siemens.de>,
	Alfred Hilscher <Alfred.Hilscher@icn.siemens.de> writes:
> 
> Ted Dennison wrote:
>> 
>> > question is obsolete. What I'm looking for is something like a
>> "packege
>> > type" (similar to "task type"). What I want do is something like this:
>> 
>> > Any suggestions how to do this ?
>> 
>> That's what generics with package formal parameters are for.
> 
> 
> I think thats not quite the same. Taking the example below, how would
> you do this with generics ?
> 
> 
> package type stack is
>   push (item : in ...);
>   pop  (item : out ...)
> end stack
> 
> procedure application is
>   User_stack : stack;
>   Supervisor_stack : stack;
>   Interrupt_stack : stack;
> 
>   Any_Stack : access Stack;
> begin
>  ...
>   Any_Stack := new Stack; -- how do this  ???
>  ...
> end application;

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.)

/Anders
-- 
"A well-written program is its own heaven; 
 a poorly-written program is its own hell."
  - The Tao of Programming