Re: Actual stream of a certain type

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

On Thu, 27 Jul 2006 00:47:00 +0100, Marius Amado-Alves wrote:

> Thanks, I know, I tried that, but I don't seem to succeed combining  
> that with providing T_Stream as a mix-in. I also tried Ada 2005  
> interfaces with GNAT. I thought I saw a real case for interfaces and  
> that interested me because all examples I have seen so far are  
> academic, not compelling. But failed. Every time Ada got in the way.  
> It's really a love-hate relation with her.

You have a "parallel types hierarchy" problem, which in general has no
solution in Ada.

On Thu, 27 Jul 2006 01:25:04 +0100, in comp.lang.ada you wrote:

> Ok, here it goes. Of all idioms I tried maybe the least unsuccessful  
> (?) ones are those that raise Program_Error with message  
> "accessibility check failed" (GNAT) for line marked HERE below. The  
> problem here is: how to use the stream visible on that line, as the  
> good T_Stream object he is, pass it as a variable, etc? Thanks a  
> great lot.
> 
>     type T_Stream_Class_Access is access all T_Stream'Class;
> 
>     procedure Write_T
>       (Stream : T_Stream_Class_Access;
>        Item : in T) is
>     begin
>        Foobar (Stream.all, Item);
>     end;
> 
>     procedure Write
>       (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
>        Item : in T) is
>     begin
>        if Stream.all not in T_Stream'Class then
>           raise Actual_Stream_Must_Be_A_T_Stream;
>        else
>           Write_T (T_Stream_Class_Access (Stream), Item); -- <== HERE
>        end if;
>     end;

I would use (double-dispatch emulation):

   procedure Write_T (Stream : in out T_Stream; Item : T'Class) is
   begin
      Foobar (Stream, Item);
   end Write_T;

   procedure Write
             (  Stream : access Ada.Streams.Root_Stream_Type'Class;
                Item   : T
             )  is
   begin
      if Stream.all not in T_Stream'Class then
         raise Actual_Stream_Must_Be_A_T_Stream;
      else
         Write_T (T_Stream'Class (Stream.all), Item);
      end if;
   end Write;

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