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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Question on initialization of packages
Newsgroups: comp.lang.ada
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Organization: cbb software GmbH
References: <y4adnXtddsFN7crV4p2dnAA@telenor.com>
Date: Tue, 17 Jun 2008 10:50:57 +0200
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Date: 2008-06-17T10:50:57+02:00
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On Tue, 17 Jun 2008 10:07:43 +0200, Reinert Korsnes wrote:

> Assume the package definition given below, 
> and the follow code in my program:
> 
>    type Message_t;
>    type Message_ta is access Message_t;
>    package Message_Stack_p is new Stacks(Message_ta);
>    Send_Stack    : Message_Stack_p.Stack;
> 
> Question: How can I be sure that "Send_Stack" is empty
> at the start of the program execution ?

Hmm, "sure" in which sense? To make it visible for the reader? To specify
in the contract of Stack that it is initially empty?

As for implementation you posted, the stack is empty, because instances of
access types are initialized with null (when not explicitly initialized
otherwise). Below you declare:

>   type Stack is access Cell;

And

   Send_Stack : Message_Stack_p.Stack; -- This will be null = empty


Now side comments:

1.

>   procedure Pop(S: in out Stack; X: out Item) is
>   begin
>     X := S.Value;
>     S := S.Next;
>   end;

This is a memory leak. If you allocated a stack element on push, you should
free it on pop.
 
2. Package initialization is achieved as follows:

package body P is
   ...
begin
   ... -- Initialize package stuff
end P;

3. Package initialization cannot help you here, because the package
declares an abstract data type of which objects can be created long after
the package itself was elaborated (initialized).

4. You have messages. Pointers to messages. These pointers are copied into
dynamically allocated linked list called stack.

How are you going to maintain this? Do you want to prevent messages from
being copied? Then you should reconsider the design of messages allowing
their queuing without stacks. Alternatively, do you want to copy messages
upon queueing (to marshal them)? Then the queue should deal with
unconstrained objects:

   generic
      type Message (<>) is private;
   package Queue is
      ...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de