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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Object-Oriented style question
Date: Sun, 8 Jan 2012 14:25:19 +0100
Organization: cbb software GmbH
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Date: 2012-01-08T14:25:19+01:00
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On Sun, 08 Jan 2012 12:52:46 +0000, Simon Wright wrote:

> I don't see why anyone would use anything other than your Info:
> 
>    function Info (Item : in T) return Value;
> 
> You may need to pass values of T'Class (or access T'Class) around, but
> this shouldn't affect the primitive. And it seems to me it makes it much
> simpler from a teaching point of view (but IANAT!)

In some rare cases Info need to be is contravariant ("final"), then:

   function Info (Item : in T'Class) return Value;

When T is supposed to use some reference counting scheme. Then due to lack
of either MI or interface inheritance in Ada, there must be an extra
interface type declared. E.g.

   type T_Interface is limited interface ...;
   function Info (Item : in T_Interface) return Value is abstract;

   type T_Implementation is ... and T_Interface;
   overriding -- This one does the job
      function Info (Item : in T_Implementation) return Value;

   type T_Handle is ... and T_Interface ...
   overriding -- This one delegates to the implementation
      function Info (Item : in T_Handle) return Value;

   function Info (Item : in T_Handle) return Value is
   begin
       return Item.Ptr.Info;
   end Info;

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