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From: Nick Roberts <nick.roberts@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: visibility of private incomplete types
Date: Sat, 30 Oct 2004 04:13:14 +0100
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Date: 2004-10-30T04:13:14+01:00
List-Id: <comp.lang.ada>

Frank J. Lhota wrote:

>>Also, would I be right in saying that, in Ada 95, it is not permitted
>>for a type declared (by an incomplete type declaration) in the private
>>part of a package to be completed in the package's body?
> 
> No, this was permitted as early as Ada 83, and I used this technique many 
> times. For example, assume that we are writing a package that implements 
> widgets. Widgets will be implemented as pointers to a record, but of course 
> we will hide this implementation detail using a private type. We can also 
> hide the structure of this record type as follows:
> 
>     package Widgets is
> 
>         type Widget_Type is limited private;
> 
>         procedure Open( Widget : in out Widget_Type; Name : in String );
>         procedure Close( Widget : in out Widget_Type );
>         ...
> 
>     private
> 
>         type Widget_Data;
>         type Widget_Type is access Widget_Data;
> 
>     end Widget;
> 
> This type of package specification in legal in both Ada 83 and Ada 95 (and 
> presumably in Ada 200x), for Widget_Data is never used in a way that 
> requires a full declaration. Of course, Widget_Data must be fully declared 
> in the package body.

I have to laugh. I now recall that I have used this technique /myself/
in the past. What a memory! RM95 3.10.1 (3). Sorry.

-- 
Nick Roberts