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Date: Thu, 13 Aug 2009 11:18:47 +0300
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Organization: Tidorum Ltd
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Newsgroups: comp.lang.ada
Subject: Re: Ada 95 and Class attribute for none-tagged incomplete type
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Date: 2009-08-13T11:18:47+03:00
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Randy Brukardt wrote:
> "Adam Beneschan" <adam@irvine.com> wrote in message 
> news:17b5de2e-74df-414a-b214-677344dc697e@x25g2000prf.googlegroups.com...
>> On Aug 1, 11:55 pm, Hibou57 (Yannick Duch�ne) <yannick_duch...@yahoo.fr> 
>> wrote:
>>> Hello,
>>>
>>> I've learned something mostly surprising today : in Ada 95, this was
>>> allowed to use the Class attribute with a prefix which was an
>>> incomplete type.... even not tagged.
> 
> Gee, guys, that's pretty boring. If you want something mind blowing in this 
> area, look into 7.3.1(8-9) 
> (http://www.adaic.org/standards/05rm/html/RM-7-3-1.html).
> 
> More than 90% of the ARG (that is, everyone other than Tucker, and he only 
> had a vague recollection of it) was unaware of this rule when I stumbled 
> across it updating notes. And no one can remember why we would want such a 
> rule - it seems completely privacy breaking, and for no good reason.

I don't understand -- 7.3.1(9) says (both in RM 95 and RM 05) that the 
'Class attribute is "allowed only from the beginning of the private part 
in which the full view is declared..."

> 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).

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