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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Self pointer in limited record
Date: Mon, 10 Sep 2007 10:13:23 -0700
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Date: 2007-09-10T10:13:23-07:00
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On Sep 10, 9:27 am, amado.al...@gmail.com wrote:
> > > > ... I wish this were
> > > > possible myself. Or more simply:
>
> > > > type T is -- limited or not
> > > >    Self : access T := T'Unchecked_Access;
> > > >    ...
> > > > end; (Marius)
>
> > > T cannot be non-limited, because otherwise passing it by copy would make
> > > rubbish out of Self. In any case it would make little sense if not access
> > > T'Class. (Kasakov)
>
> > Sure it can... (Randy)
>
> Yes, now the compiler is accepting this (and letting me copy objects,
> and, yes, getting rubbish in Self). I have no idea why it was not
> compiling before. It just wasn't. And the messages indicated trouble
> in the type. So, and because I know that Ada is a very sensitive lady,
> I jumped to the (wrong) conclusion that the thing was illegal.

If the compiler is letting you compile this:

    type T is record
       Self : access T := T'Unchecked_Access;
    end record;

then the compiler is wrong, because this is illegal.  The prefix of
'Unchecked_Access must be aliased, and the current instance of a non-
limited record is not aliased.  (Using 'Unchecked_Access instead of
'Access eliminates accessibility level checks, but the other rules
that apply to 'Access, including the rule that it may apply only to an
aliased view.  See 3.10.2(23), 3.10(9) (and AARM 3.10(9.b)),
13.10(2-3).  Using 'Unrestricted_Access may be OK, though; it's not
part of the language, and GNAT gets to make whatever rules they like
about whether it's legal.

                        -- Adam