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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: orthogonal inheritance and extension aggregates
Date: Sat, 20 Jul 2013 10:54:44 +0300
Organization: Tidorum Ltd
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Date: 2013-07-20T10:54:44+03:00
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On 13-07-20 09:36 , Dmitry A. Kazakov wrote:
> On Sat, 20 Jul 2013 00:49:41 -0500, Randy Brukardt wrote:
> 
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
>> news:igo2b70i4ox.t5duix9weguc$.dlg@40tude.net...
>> ...
>>> P.S. I don't know if Ada 2012 silently inherits from null extension. I
>>> heard that there was an intention to add the kludge. Though semantically
>>> broken it would save programs like yours.
>>
>> That's actually an Ada 2005 feature - see 3.9.3(4/2). One that I was against 
>> because it introduces a maintenance problem (null extensions tend to turn 
>> into non-null extensions over time, and once that happens, the feature stops 
>> working).
> 
> Right.
> 
>> It appears that GNAT failed to properly implement this feature in the case 
>> described by the OP.
> 
> But the extension is null only *privately*.

The *private* extension looked non-null to me in the original post,
where an ellipsis marks the extension in the private part of the generic:

On 13-07-19 20:46 , Felix Krause wrote:
> I try to implement some feature via orthogonal inhertance like this:
>
>    generic
>       type Parent (<>) is abstract tagged private;
>    package Extension is
>       type Extended is abstract new Parent with private;
>
>       -- operations of the extended type here
>    private
>       type Extended is abstract new Parent with record
>          -- …
>       end record;
>    end Extension;

The visible extension to a non-abstract type was null:

>    type Child is new Base.Extended with null record;

It seems to me that GNAT is working correctly on this code, assuming
that the private extension is not null.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
      .      @       .