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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: abstract types and subprograms
Date: Tue, 20 May 2014 16:11:53 +0200
Organization: cbb software GmbH
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Date: 2014-05-20T16:11:53+02:00
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On Tue, 20 May 2014 15:28:29 +0300, Victor Porton wrote:

> Dmitry A. Kazakov wrote:
> 
>> function From_Handle(Handle: Dummy_Record_Access) return
>> Base_Object'Class;
>> 
>> Logically, a handle can point on an object of any type derived from
>> Base_Object, so it is a class.
> 
> But can this:
> 
>    function From_Handle(Handle: Dummy_Record_Access) return Base_Object is
>       (Ada.Finalization.Limited_Controlled with Handle=>Handle);
> 
> be implemented if the return type changes to Base_Object'Class?

It does not make sense to me. Probably because it is not clear what you are
going to achieve, smart pointer, an opaque handle to a C object, no idea.

> It seems that returning Base_Object'Class wouldn't magically make it work 
> (without explicit override).

Either you override or you don't. As I said, the criterion is whether the
implementation is same for all class or different for some instances.
 
> Or can it be done implementing Frorm_Handle only once (for Base_Object) with 
> automatic changing its signature for derived tagged types?

Signature changes if the operation is primitive. It does not change if it
is not primitive, e.g. class-wide.

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