From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,23cf9f1e93744eed
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2003-07-30 15:45:05 PST
Path: 
 archiver1.google.com!news1.google.com!sn-xit-02!sn-xit-06!sn-post-01!supernews.com!corp.supernews.com!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Need advice re package organization.
Date: Wed, 30 Jul 2003 17:46:48 -0500
Organization: Posted via Supernews, http://www.supernews.com
Message-ID: <vigijhb0op0t69@corp.supernews.com>
References: <pan.2003.07.26.13.08.25.810488@mail.utexas.edu>
 <3F228F3B.9020203@attbi.com> <yaxUa.374$XK4.16279@read1.cgocable.net>
 <3F22F9E9.3040307@attbi.com> <M2YUa.972$XK4.65085@read1.cgocable.net>
 <5jn9ivoetll1fu2avn9hmjj6aaa7q7pmjn@4ax.com>
 <y%xVa.1302$j_5.4631@news20.bellglobal.com>
 <fiveiv8h309dkvjqf6nb343osfjl41qt3o@4ax.com>
 <clWVa.4191$mv6.741217@news20.bellglobal.com>
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4910.0300
X-Complaints-To: abuse@supernews.com
Xref: archiver1.google.com comp.lang.ada:41047
Date: 2003-07-30T17:46:48-05:00
List-Id: <comp.lang.ada>

"Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca> wrote in message
news:clWVa.4191$mv6.741217@news20.bellglobal.com...
> The problem occurs in the private child package (IMO) because
> you want to derive new objects from parent tagged records. The
> danger that develops is that the child package's body code may
> monkey with things in the parent object that you don't want it
> to (this is lack of information hiding).
>
> However, you can fix the object visibility by making it a non
> child package (just another package using the first). However,
> the problem then becomes that you lose access to other types
> and definitions that are in the other package's private part.
>
> So in my mind, there still exists a short-coming OO wise
> in the visibility rules for objects: it needs a way to
> mark parts of a tagged object as "off limits" for the child
> package, which always enjoys full visibility of the parent
> package's tagged record (object).

You can do this by deferring the contents of the tagged type to the body:

     type My_Type_Contents;
     type My_Type_Contents_Access is access all My_Type_Contents;
     type My_Type is tagged record
          Contents : My_Type_Contents_Access;
     end record;

Then no child can see the contents. Claw uses this technique in some cases.
The body contains the full declaration of My_Type_Contents.

This technique works best when the tagged type is Controlled (since the vast
majority of tagged types ought to be controlled, this is not a real
restriction). Then the Initialize routine can allocate the contents, and the
Finalize can free it. The only remaining danger would be someone assigning a
Null into the Contents access, which is a lot less likely than fiddling with
components.

               Randy.