On Wed, 7 Feb 2001 23:52:36 +0100, "Jean-Pierre Rosen"
<rosen.adalog@wanadoo.fr> wrote:

>
>"Ted Dennison" <dennison@telepath.com> a �crit dans le message news: 95rpj5$gq7$1@nnrp1.deja.com...
>> I have to admit I'm a little unclear as to what nasty things could
>> happen to me if I were allowed to declare a derived tagged type at a
>> lower scope level than their parent type.
>OK, let's go:
>
>package body Pack is
>   type Parent is tagged ...;
>   procedure Proc (X : Parent) is ... end Proc;
>
>   type Ptr is access Parent'Class;
>   V : Ptr;
>
>   procedure P is
>      type Child is new Parent with...;
>      procedure Proc (X : Child) is... end Proc;
>   begin
>      V := new Child;
>   end P;
>
>begin
>   P;
>-- Now, V.all points to an object of type P.Child
>-- We can call Proc (V.all) which will result in a dispatching call to P.Proc, but P is out of scope at this point !!!

Dabbling in language design is fun :-)

It seems to me that this case could be handled by the already existing
accessibility level rules for access types. After all, type Child has
a level that is statically deeper than that of type Ptr, so the
assignment might simply be disallowed. Of course, one could still play
tricks with 'Unchecked_Access, but then, it's always been the
developers' responsibility to get things right when using
'Unchecked_Access!

As to P.Proc being out of scope: I guess this could be detected at
run-time. At least an implementation using static links could detect
that the static link in a call Proc (V.all) is null, and raise an
exception. A similar solution probably also exists for implementations
using displays.

I see a more serious problem with

package body Pack is
   type Parent is tagged ...;
   
   function Create return Parent'Class
   is
      type Child is new Parent with ...;
      Result : Child;
   begin
      return Result;
   end;

   P : Parent'Class := Create;

end Pack;

But even this could perhaps be handled by extending the accessibility
rule to class-wide types, too.

Are you aware that there is an AI on downward closures for subprogram
access types and limited tagged types (AI-00254)? It seems to me that
this AI has some relevance to this issue.

As for downward closures on subprogram access types: IIRC, Oberon
(from Wirth at ETH Zurich) had this.


-- 
Dr. Thomas Wolf <t_wolf@my-deja.com>