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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 13:55:25 +0200
Organization: cbb software GmbH
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Date: 2014-05-20T13:55:25+02:00
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On Tue, 20 May 2014 13:22:30 +0300, Victor Porton wrote:

> By the philosophy of programming From_Handle should be non-abstract but 
> return an abstract object, and From_Handle should be automatically 
> overridden for descendants non-abstract objects.
> 
> What is wrong?

You cannot automatically override without knowing the implementation, which
you don't in advance. If you know it, then it is same for all class. See
below.

> -- rdf-base.ads
> with Ada.Finalization;
> with Interfaces.C.Pointers;
> 
> package RDF.Base is
> 
>    -- Internal
>    type Dummy_Record is null record;
> 
>    -- Internal
>    type Dummy_Record_Access is access Dummy_Record;
>    
>    type Base_Object is abstract new Ada.Finalization.Limited_Controlled with 
> private;
>    
>    function Get_Handle(Object: Base_Object) return Dummy_Record_Access with 
> Inline;
>    
>    function From_Handle(Handle: Dummy_Record_Access) return Base_Object with 
> Inline;

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.

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