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=-0.3 required=5.0 tests=BAYES_00,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Thread: 103376,12a63150f4f961a
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Received: by 10.68.191.225 with SMTP id hb1mr1418350pbc.5.1336218850174;
        Sat, 05 May 2012 04:54:10 -0700 (PDT)
Path: 
 pr3ni6690pbb.0!nntp.google.com!news2.google.com!volia.net!news2.volia.net!feed-A.news.volia.net!news.musoftware.de!wum.musoftware.de!news.mixmin.net!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: OOP in Ada: Alternatives for "protected" scope
Date: Sat, 5 May 2012 13:53:59 +0200
Organization: cbb software GmbH
Message-ID: <hu3l1msk42yo$.14kbjfv94ermy$.dlg@40tude.net>
References: <jnut8r$aqb$1@online.de>
 <1vn9nv4aqm9a3.1tqzc91idjlbs$.dlg@40tude.net> <jo18al$f4m$1@online.de>
 <17kcn9g4qeg7j.5xole2f4bwbj$.dlg@40tude.net> <jo2obr$tln$1@online.de>
Reply-To: mailbox@dmitry-kazakov.de
NNTP-Posting-Host: KA5xyyPKkGZLxk9XJsNqLQ.user.speranza.aioe.org
Mime-Version: 1.0
X-Complaints-To: abuse@aioe.org
User-Agent: 40tude_Dialog/2.0.15.1
X-Notice: Filtered by postfilter v. 0.8.2
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Date: 2012-05-05T13:53:59+02:00
List-Id: <comp.lang.ada>

On Sat, 5 May 2012 10:28:43 +0200, Felix Krause wrote:

> Yes, but that's not what I wanted. Let's have a more specific example 
> using orthogonal inheritance:
> 
> package P is
>    type A is tagged private;
>    -- ...
> end P;
> 
> generic
>    type Base is new A with private;
> package P.Extend is
>    type Extended is new Base with private;
>    -- ...
> end P.Extend;
> 
> package P.Q is
>    type B is new A with private;
>    -- ...
> end P.Q;
> 
> package P.Q.R is
>    package B_Extend is new P.Extend (Base => B);
>    type C is new B_Extend.Extended with private;
> end P.Q.R;
> 
> (I actually have some code with this structure.) I cannot possibly 
> access the private part of P.Extend in P.Q.R - although I'm extending 
> the class defined there.

Extending = use. Using does not give special rights on the content! (:-))
The idea is that you create derived types in a way independent on the
implementation of the parent. If the provider of the parent hid some stuff
in the private part, there must be a good reason why nobody should look
after.

BTW, things hidden in the bodies are inaccessible by any means. My attitude
to "private" is to treat it as a compromise helping the compiler to know
what it should know, rather than a design tool to give access to children.
Ideally, the private part should contain only things which cannot be moved
to the body due to language constraints.

> I cannot layout the packages in a way that 
> lets me access both P.Extend's and P.Q's private part in P.Q.R.

Are you constructing the dreadful Rhombus? (:-))

If a package need to see internals of two packages it must be a descendant
of both. This makes it generic if one of them is.

   package P is
      type A is tagged private;
      -- ...
   end P;

   package P.Q is
   type B is new A with private;
      -- ...
   end P.Q;

   generic
      type Base is new A with private;
   package P.Q.Extend is
      type Extended is new Base with private;
      -- ...
   end P.Q.Extend;

   generic
   package P.Q.Extend.R is
      type C is new Extended with private;
   end P.Q.Extend.R;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de