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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Self pointer in limited record
Date: Fri, 31 Aug 2007 10:37:10 -0700
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Date: 2007-08-31T10:37:10-07:00
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On Aug 31, 9:47 am, amado.al...@gmail.com wrote:
> On 4 Jul, 20:31, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>
> > Consider:
>
> > type T_Access is access all T;
> > type T is  new Ada.Finalization.Limited_Controlled with record
> >    Self: T_Access := T'Unchecked_Access;
> >    -- more components
> >    -- ...
> > end record;
>
> > I have seen this pattern repeatedly.
>
> Then you have seen illegal Ada code repeatedly. I wish this were
> possible myself. Or more simply:
>
> type T is -- limited or not
>    Self : access T := T'Unchecked_Access;
>    ...
> end;
>
> But the compiler will remind you that Unchecked_Access is not
> available for types.

No, this can be legal.  Normally, you can't apply 'Unchecked_Access to
a type name.  But within the definition of a type T, the use of T in a
context like this refers to the "current instance"; that is, it will
refer to whatever object is declared with that type.  So if you later
declare "X : T;", then the T in T'Unchecked_Access will be replaced by
X for that declaration (and X.Self will thus point to X).  See
8.6(17).  However, it's only legal if T is limited (in Ada 2005, the
rule is slightly more restrictive), because 'Unchecked_Access can only
be applied to an aliased entity, and 3.10(9) says that the current
instance of a *limited* type is defined to be aliased.

                  -- Adam