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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 95 and Class attribute for none-tagged incomplete type
Date: Thu, 13 Aug 2009 09:16:29 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2009-08-13T09:16:29-04:00
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Niklas Holsti <niklas.holsti@tidorum.invalid> writes:

> Randy Brukardt wrote:
>> A quick example:
>> package P is
>>    type Priv is private;
>>    type Acc is access all Priv'Class; -- Legal!!!!!!
>> private
>>    type Priv is tagged null record;
>> end P;
>
> This conflicts with 7.3.1(9) because Priv'Class is used before the
> private part. Gnat (Debian Lenny) says:
>
> p.ads:3:27: tagged type required, found private type "Priv" defined at
> line 2
>
> If the declaration of type Acc is moved below "private", Gnat accepts
> the code, as it should by 7.3.1(8-9).

I agree with Niklas.  The above is illegal.

The purpose of the rule is so you can create recursive types, as in:

package P is
   type Priv is private;
private
   type Acc is access all Priv'Class; -- Legal!!!!!!
   type Priv is tagged
        record
            Next : Acc;
            ... --etc.
        end record;
end P;

without exposing the taggedness to clients.  Note that Acc is in the
private part, so there's no privacy-breaking going on.

- Bob