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: 103376,174ec7dc941a1068
X-Google-Attributes: gid103376,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news2.google.com!news1.google.com!news.glorb.com!feeder1-2.proxad.net!proxad.net!feeder2-2.proxad.net!newsfeed.arcor.de!newsspool2.arcor-online.net!news.arcor.de.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Factory Pattern
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: <1185387571.367570.163160@r34g2000hsd.googlegroups.com>
Date: Wed, 25 Jul 2007 20:51:12 +0200
Message-ID: <ehg8y50sqs84$.2grfhftszzrg$.dlg@40tude.net>
NNTP-Posting-Date: 25 Jul 2007 20:51:07 CEST
NNTP-Posting-Host: f2d57f43.newsspool1.arcor-online.net
X-Trace: 
 DXC=Vk8`FcbeG9deoCI^f\Y]Eaic==]BZ:afn4Fo<]lROoRaFl8W>\BH3YbiG79[537SkdDNcfSJ;bb[eIRnRBaCd<MnX<hUC\;66YbkNiZKBl[Y1n
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news2.google.com comp.lang.ada:1180
Date: 2007-07-25T20:51:07+02:00
List-Id: <comp.lang.ada>

On Wed, 25 Jul 2007 11:19:31 -0700, shaunpatterson@gmail.com wrote:

> I'm just starting out learning Ada and I'm having some trouble
> converting some C++ code.
> 
> The last hurtle I'm having trouble with is implementing a Factory in
> Ada
> 
> I basically have a base class Message and all other messages are
> derived from this.
> I have a function call createMessage that I would like to return the
> appropriate subclass type.
> 
> My code looks something like this:
> 
> type Message is abstract tagged;

   -- Abstract factory, each non-abstract derived type will be forced to
   -- implement this:
function Create (I : Integer) return Message is abstract;

> type Message_Class is access all Message'Class;
> 
> derived type:
> 
> package MessageInteger_Pkg is
> 
> -- I want this to act like a constructor
> -- Just fills in the value
> function create (i : Integer) return MessageInteger;
> 
> private
> type MessageInteger with new Message with
> record
>       value : Integer;
> end record;
> 
> 
> end MessageInteger_Pkg;
> 
> -- create looks like this:
> function create (i : Integer) return MessageInteger is
>      msg : MessageInteger;
> begin
>      msg.value := i;
>      return msg;

You could use an extension aggregate here:

   return (Message with I);

> end create;
> 
> Factory method:
> 
> -- I would like it to work like this...
> function createMessage (type : Integer) return Message_Class is

"Type" is a reserved word.

> begin
>          case type is
>                  when 0 =>
>                       return MessageInteger_Pkg.create (1234);
>                  when others =>
>                        return MessageSomethingElse_Pkg.create
> ("blah");
>          end case
> 
> end createMessage

  function Factory (What : Integer) return Message'Class is
   begin
      case What is
         when 0 => return MessageInteger_Pkg.Create (1234);
         ...

> However - it doesn't like me returning the MessageInteger from a class
> that returns a Message_Class (obviously..)

[...]
> Perhaps I should move the "new"
> to the create function?

Yes, you you want to work with pointers. But what for? Ada can return
polymorphic objects on the stack. No need in pointers.

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