Re: Limited returns

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 23 Jun 2008 08:15:12 -0700 (PDT), Adam Beneschan wrote:

> On Jun 23, 7:21 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> Let's consider this:
>>
>>    with Ada.Finalization;
>>    procedure Test_Limited_Stuff is
>>       type I is limited interface;
>>
>>       type T is new Ada.Finalization.Limited_Controlled with record
>>          A : Integer;
>>       end record;
>>
>>       type S is new T and I with record
>>          Self : not null access S'Class := S'Unchecked_Access;
>>       end record;
>>
>>       function Factory return I'Class is
>>       begin
>>          return X : S;
>> -- GNAT: wrong type for return_subtype_indication
>>       end Factory;
>>    begin
>>       null;
>>    end Test_Limited_Stuff;
>>
>> Is it illegal? I am surprised why? S is in I'Class.
> 
> It's illegal, but hopefully not for long.  See:
> 
> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AI05s/AI05-0032-1.TXT?rev=1.4

It would be nice, because this "feature" is extremely boring.

> Perhaps you could ask your compiler to go ahead and implement that AI
> (or at least the part of it that allows the specific return type as
> you've done).  I think it was just an oversight in Ada 2005 not to
> allow this.

Is there an AI to finally fix a similar problem with pool-specific access
types? I mean this:

procedure Test_Access is
   type T is tagged null record;
   type S is new T with null record;
   
   type T_Ptr is access T'Class;
   type S_Ptr is access S'Class;

   function Factory return T_Ptr is
      X : S_Ptr := new S;
   begin
      .. -- Do something with X
      return T_Ptr (X); -- Illegal!???
   end Factory;
begin
   null;
end Test_Access;

The only workaround is to cast:

   function Factory return T_Ptr is
      Result : P_Ptr := new S;
      X : S'Class renames S'Class (Result.all);
   begin
      .. -- Do something with X
      return Result;
   end Factory;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de