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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Strange error message
Date: Sun, 1 Jun 2014 13:11:08 +0200
Organization: cbb software GmbH
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Date: 2014-06-01T13:11:08+02:00
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On Sun, 1 Jun 2014 03:24:02 -0700 (PDT), Charly wrote:

> when I defined a class hierarchy I encountered a problem, that I could reduce 
> to the following few lines:
> 
> This version compiles without problems:
> -----------------
> package Test is
> 
>    type Base is tagged private;
>    function Create return Base;
> 
>    type High is new Base with private;

You must override Create for High, *always*.
 
> Why do I have to overide the function Create for type High.
> I don't see the necessity.
> 
> Btw. I'm using GNAT GPL 2014 but the same happens with 2013 version.

I believe it was a recent hack (wrongly) added to Ada that you need not to
override primitive operations returning the tagged type when the derived
type does not actually extend. I don't know if making that public or
privately changes anything. You did that privately.

It is an awful idea in any form. You should always override such
operations, and all out-operations, because inheriting them is a priori
unsafe.

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