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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Question on initialization of packages
Date: Tue, 17 Jun 2008 10:29:12 -0400
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Date: 2008-06-17T10:29:12-04:00
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Reinert Korsnes <a@b.no> writes:

> I try to use a stack in my Ada program.
>
> 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 ?
>
> How can I make explicite that "Send_Stack" is empty in this case ?

Here's one way:

Declare Stack like this:

    type Stack(<>) is limited private;

The "(<>)" means clients cannot declare objects without initializing
them.  The "limited" means clients cannot copy them.

Declare the full type like this:

    type Stack is limited
        record
            ... -- pointer to first cell, and count can go here
        end record;

Declare a constructor function:

    function Empty_Stack return Stack;

Declare other constructor functions if you want.

Then:

    Send_Stack: Message_Stack_p.Stack := Message_Stack_p.Empty_Stack;

Note that this works only in Ada 2005 mode.

- Bob