Re: Dynamic allocation of unconstrained types

[Adam Beneschan <adam@irvine.com>]

On Sep 30, 7:29 am, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> Consider:
>
> procedure Test is
>
>    package P is
>       type T (<>) is limited private;
>       function Create return T;
>    private
>       type T is limited record
>          I : Integer;
>       end record;
>    end P;
>
>    package body P is
>       function Create return T is
>       begin
>          return T'(I => 123);
>       end Create;
>    end P;
>
>    S : access P.T;
>
> begin
>    S := new P.T'(P.Create);   -- ??? (this is line 22)
> end Test;
>
> GNAT says:
>
> test.adb:22:19: uninitialized unconstrained allocation not allowed
> test.adb:22:19: qualified expression required
>
> Interestingly, it works with Strings.
> Why doesn't GNAT recognize it as a qualified expression?

It looks like a bug to me.  I do notice that if you remove "limited"
from both declarations of T, then it works.  There are some new Ada
2005 rules that allow limited-type expressions in more places
(functions returning limited types weren't allowed in Ada 95), so it's
likely that there was a mistake in implementing this new feature.


> I would like to allocate dynamically something that has a constructor
> function. There is no other way to create the object than with that
> function and presumably it should be possible to use it with dynamic
> allocation.
> How can I do it?
>
> BTW - when preparing this example I tried first with empty (null)
> record, but got stuck with proper way to return an instance of T. I
> remember there was some older discussion about it, but for some reason
> I cannot find it and the following:
>
> return T'(others => <>);
>
> is rejected as well.

Try

   return T'(null record);

The first attempt, with (others => <>), should have worked, but there
was faulty language in the RM that made this illegal for a null
record.  It has been fixed by a Binding Interpretation, AI05-16, so I
think compilers should allow that construct, but perhaps it hasn't yet
been fixed in the version of GNAT you're using.

                                   -- Adam