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,e9247d4ba8198ede
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news2.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!newscon06.news.prodigy.com!prodigy.net!newspeer.monmouth.com!newsfeed.icl.net!newsfeed.fjserv.net!newsfeed.ision.net!newsfeed2.easynews.net!ision!newsfeed.arcor.de!news.arcor.de!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Actual stream of a certain type
Newsgroups: comp.lang.ada
User-Agent: 40tude_Dialog/2.0.15.1
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Reply-To: mailbox@dmitry-kazakov.de
Organization: cbb software GmbH
References: <mailman.0.1153934868.32700.comp.lang.ada@ada-france.org>
 <yoqdnX8teKA4LlrZnZ2dnUVZ_rWdnZ2d@comcast.com>
 <mailman.2.1153957636.32700.comp.lang.ada@ada-france.org>
Date: Thu, 27 Jul 2006 10:05:44 +0200
Message-ID: <kqdop3ewo06.z30ns7l2w4h5.dlg@40tude.net>
NNTP-Posting-Date: 27 Jul 2006 10:05:44 MEST
NNTP-Posting-Host: e06f643d.newsread2.arcor-online.net
X-Trace: 
 DXC=5_nA]61Q?k53JZ<GaG=_E2Q5U85hF6f;4jW\KbG]kaM8U7^]5?JhlB>2f42[h8HlV5[6LHn;2LCV>7enW;^6ZC`4IXm65S@:3>?
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news2.google.com comp.lang.ada:5960
Date: 2006-07-27T10:05:44+02:00
List-Id: <comp.lang.ada>

On Thu, 27 Jul 2006 00:47:00 +0100, Marius Amado-Alves wrote:

> Thanks, I know, I tried that, but I don't seem to succeed combining  
> that with providing T_Stream as a mix-in. I also tried Ada 2005  
> interfaces with GNAT. I thought I saw a real case for interfaces and  
> that interested me because all examples I have seen so far are  
> academic, not compelling. But failed. Every time Ada got in the way.  
> It's really a love-hate relation with her.

You have a "parallel types hierarchy" problem, which in general has no
solution in Ada.

On Thu, 27 Jul 2006 01:25:04 +0100, in comp.lang.ada you wrote:

> Ok, here it goes. Of all idioms I tried maybe the least unsuccessful  
> (?) ones are those that raise Program_Error with message  
> "accessibility check failed" (GNAT) for line marked HERE below. The  
> problem here is: how to use the stream visible on that line, as the  
> good T_Stream object he is, pass it as a variable, etc? Thanks a  
> great lot.
> 
>     type T_Stream_Class_Access is access all T_Stream'Class;
> 
>     procedure Write_T
>       (Stream : T_Stream_Class_Access;
>        Item : in T) is
>     begin
>        Foobar (Stream.all, Item);
>     end;
> 
>     procedure Write
>       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
>        Item : in T) is
>     begin
>        if Stream.all not in T_Stream'Class then
>           raise Actual_Stream_Must_Be_A_T_Stream;
>        else
>           Write_T (T_Stream_Class_Access (Stream), Item); -- <== HERE
>        end if;
>     end;

I would use (double-dispatch emulation):

   procedure Write_T (Stream : in out T_Stream; Item : T'Class) is
   begin
      Foobar (Stream, Item);
   end Write_T;

   procedure Write
             (  Stream : access Ada.Streams.Root_Stream_Type'Class;
                Item   : T
             )  is
   begin
      if Stream.all not in T_Stream'Class then
         raise Actual_Stream_Must_Be_A_T_Stream;
      else
         Write_T (T_Stream'Class (Stream.all), Item);
      end if;
   end Write;

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