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From: christoph.grein@eurocopter.com
Newsgroups: comp.lang.ada
Subject: Re: Question on initialization of packages
Date: Tue, 17 Jun 2008 03:18:22 -0700 (PDT)
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On 17 Jun., 10:50, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> 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:
>
> > =A0 type Stack is access Cell;
>
> And
>
> =A0 =A0Send_Stack : Message_Stack_p.Stack; -- This will be null =3D empty

But N is undefined, so you have an empty stack with an undefined
number of elements.

There is no way to make it visible to users that the stack is empty
after declaration of a stack object if you hide the implementation.
You must state this as a comment; this belongs to the contract of the
ADT and you must implement it fulfilling the contract:

private
  type Cell;
  type Stack is access Cell;
  type Cell is record
    Next : Stack;
    N    : Integer :=3D 0;  -- better use Natural here, or will you ever
have a negative number of elements?
    Value: Item;
  end record;
end Stacks;

Your stack should be limited private - or do you want to copy stack
objects? Then you must be careful that deep copies are made. With this
design, you have no control over copying. A copy of a stack will be a
shallow copy.

For freeing cells, see Ada.Unchecked_Deallocation.