Re: Question on initialization of packages

[christoph.grein@eurocopter.com]

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:
>
> >   type Stack is access Cell;
>
> And
>
>    Send_Stack : Message_Stack_p.Stack; -- This will be null = 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 := 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.