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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada design bug or GNAT bug?
Date: Sat, 4 Jul 2015 19:53:20 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:26617
Date: 2015-07-04T19:53:20-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1myo335gjwyy3$.1bthq0cugahll.dlg@40tude.net...
> On Fri, 3 Jul 2015 22:11:18 -0500, Randy Brukardt wrote:
...
>> But of course that breaks privacy: T4 knows nothing about the full type 
>> of
>> T3 or any private operations it may have.
>
> There are a lot of other cases where you effectively unable to derive from
> a type. Since they are not counted privacy breach, this one shouldn't
> either.

There are? I can't think of any. Ada 95 had an accessibility check, but 
that's gone in more recent versions of the language.

Of course, I hardly ever use derived types other than for extension. Like 
interfaces, the other uses come up so rarely that they aren't worth worrying 
about. So I may have just forgotten something.

> In my view the problem is not in T4 but in T3, like in all other cases 
> when
> you would not be able to derive later. So my solution would be a mandatory
> declaration of the public view as final. E.g.
>
> package P2 is
>   type T3 is new T1 with private; -- Illegal, no derived types possible
> private
>   type T3 is new T2 with null record;
> end P2;
>
> package P2 is
>   type T3 is new T1 with private
>       with Childless => True; -- Legal, this is the last public descendant
> private
>   type T3 is new T2 with null record;
> end P2;
>
> Now, T4 cannot claim knowing nothing.

Fine idea for Ada 9x, but way too late nowdays. We're not going to break 50% 
of the existing Ada code...

                                             Randy.