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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Suggestion: Allow functions returning abstract types in certain
 situations
Date: Thu, 22 May 2014 09:22:36 +0200
Organization: cbb software GmbH
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Date: 2014-05-22T09:22:36+02:00
List-Id: <comp.lang.ada>

On Wed, 21 May 2014 19:19:21 -0400, Robert A Duff wrote:

> Adam Beneschan <adambeneschan@gmail.com> writes:
> 
>> Hmmm... I'd worry that if we allowed an object with an abstract-type
>> tag to exist even for a little while, something in the language
>> semantics might require Initialize, Adjust, or Finalize to be called
>> on it, which could be bad if, for example, it tried to redispatch to
>> an abstract body.
> 
> Not sure what you mean about "redispatch".  You can't [re]dispatch
> without converting to class-wide.

Actually you can:

1. The Rosen's trick. You put a class-wide access discriminant into the
abstract type and dispatch on it.

2. Consider:

   type T is tagged ...;  -- Non-abstract
   type S is abstract new T with ...; -- Abstract

You convert to T, then that to T'Class and then dispatch on that. Never
needed S'Class to make it crash.

Morale. Type semantics must be respected.

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