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=unavailable autolearn_force=no version=3.4.4
Path: 
 backlog3.nntp.ams3.giganews.com!backlog3.nntp.ams.giganews.com!border1.nntp.ams.giganews.com!nntp.giganews.com!newsfeed.fsmpi.rwth-aachen.de!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Natasha Kerensikova <lithiumcat@instinctive.eu>
Newsgroups: comp.lang.ada
Subject: How to hide inherited implementation of a public interface?
Date: Fri, 21 Mar 2014 12:53:45 +0000 (UTC)
Organization: A noiseless patient Spider
Message-ID: <slrnliodi3.i0l.lithiumcat@nat.rebma.instinctive.eu>
Injection-Date: Fri, 21 Mar 2014 12:53:45 +0000 (UTC)
Injection-Info: mx05.eternal-september.org;
 posting-host="76a49b86bc3e16725b7cfca3d85cb4c8";
 logging-data="4071"; mail-complaints-to="abuse@eternal-september.org";
 posting-account="U2FsdGVkX19Obd/C4nIbZOo7gw+uw8cJ"
User-Agent: slrn/1.0.1 (FreeBSD)
Cancel-Lock: sha1:R70vA8ZIomOxeq66/5I9YDFViGw=
Xref: number.nntp.dca.giganews.com comp.lang.ada:185261
Date: 2014-03-21T12:53:45+00:00
List-Id: <comp.lang.ada>

Hello,

the subject contains my whole question, but since I'm afraid it might be
ambiguous, let's consider the basic code below:

   package I is
      type T is interface;
      procedure P (Object : in out T) is abstract;
   end I;

   package A is
      type T is abstract new I.T with private;
      function F (Object : in T) return Integer is abstract;
      overriding procedure P (Object : in out T);
   private
      type A is abstract new I.T with record
         Value : Integer;
      end record;
   end A;

   package body A is
      overriding procedure P (Object : in out T) is
         Object.Value := F (T'Class (Object));
      end P;
   end A;

   package C is
      type T is new I.T with private;
      --  What here?
   private
      type T is new A.T with record
         High : Boolean;
      end record
      overriding function F (Object : in T) return Integer;
   end C;

   package body C is
      overriding function F (Object : in T) return Integer is
      begin
         case Object.High is
            when True => return 1;
            when False => return -1;
         end case;
      end F;
   end C;

There is a concrete type C.T, that publicly provides interface I.T, but
internally uses inheritance to implement the interface. The fact that
A.T is abstract is not directly relevant, I believe the situation is
exactly the same with a concrete A.T, but it leaks the motivation behind
such a construction.

I could publicly derive C.T from A.T, and then everything would be fine.

However in the particular case I encountered, it bothers me a bit to
expose publicly that C.T is derived from A.T, since it really is an
implementation detail. The only piece of information available to
clients of C is supposed to be that I.T is implemented, no matter how.

So is there a way to publicly provide I.T while privately inheriting
almost everything from A.T?


Thanks for your help,
Natasha