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From: Marius Amado-Alves <marius@amado-alves.info>
Subject: Re: Actual stream of a certain type
Date: Thu, 27 Jul 2006 01:25:04 +0100
To: comp.lang.ada@ada-france.org
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Date: 2006-07-27T02:25:23+02:00

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;