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From: AdaMagica <christ-usch.grein@t-online.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2005 puzzle
Date: Sat, 28 Jul 2012 02:48:43 -0700 (PDT)
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Date: 2012-07-28T02:48:43-07:00
List-Id: <comp.lang.ada>

On Friday, July 27, 2012 3:04:12 PM UTC+2, Dmitry A. Kazakov wrote:
> If you cannot derive? Primitive operations make little sense if there is
> only one type.
>
> Abstract types fall short either because you could not have:
>
>    type T is abstract ... with private;
>    function Create (...) return T; -- Non-abstract
>       -- initializes private parts of T, called from the overriding
>
> The stuff just does not make any sense to me.

I see what you mean. But since no objects of abstract types may exist and s=
omehow you have to provide the private components of derived nonprivate obj=
ects, the proper Ada way (as intended by ARG, I guess) is child packages. T=
hey can see the private part and provide the necessary information. (I'm su=
re you know this.)
So you *can* derive:

package Root is
  type T is abstract tagged private;
private
  ...
end Root;
package Root.Child is  -- body can see private part of Root
  type S is new T with private;
  function Create (...) return S;
private
  ...
end Root.Child;

-- I guess that's what you want:
with Root;
package Derived is  -- this does not work, there's no way to provide the
                    -- private part of T.
  type S is new Root.T with private;
  function Create (...) return S;