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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Unchecked_Conversion and task pointer.
Newsgroups: comp.lang.ada
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Date: Thu, 7 Jul 2005 22:18:00 +0200
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On 7 Jul 2005 08:50:55 -0700, e.coli wrote:

> look at this exemple:
> ----------------------------------------------------
> with Ada.Unchecked_Conversion;
> with Ada.Text_Io;
> 
> procedure Main is
> 
>    task type Cain is
>       entry Who_Are_You;
>    end Cain;
> 
>    task type Abel is
>       entry Who_Are_You;
>    end Abel;
> 
>    type Ptr_Cain is access Cain;
>    type Ptr_Abel is access Abel;
> 
>    task body Cain is
>    begin
>       loop
>          accept Who_Are_You;
>          Ada.Text_Io.Put_Line(Item => "I'm Cain");
>       end loop;
>    end Cain;
> 
> 
>    task body Abel is
>    begin
>       loop
>          accept Who_Are_You;
>          Ada.Text_Io.Put_Line(Item => "I'm Abel");
>       end loop;
>    end Abel;
> 
> 
>    function Mess_Up is
>    new Ada.Unchecked_Conversion
>       (
>       Source => Ptr_Cain,
>       Target => Ptr_Abel);
> 
>    X : Ptr_Cain;
>    Y : Ptr_Abel;
> 
> begin
>    X:= new Cain;
>    Y:= new Abel;
>    X.Who_Are_You;
>    Y.Who_Are_You;
>    Y:=Mess_Up(X);
>    -- where are your brother?
>    X.Who_Are_You;
>    Y.Who_Are_You;
> end Main;
> ------------------------------------------------
> 
> how work Unchecked_Conversion with task pointer?

Better not to know... (:-))

> the behavior depend by the compiler?

Task types are not tagged in Ada (alas). So if you are trying to have a
kind of task types hierarchy you should use mix-in. Example:

with Ada.Unchecked_Deallocation;

package Persons is
   type Corpse is abstract tagged null record;
   function Get_Name (Context : Corpse) return String is abstract;

   task type Soul (Context : access Corpse'Class) is
      entry Who_Are_You;
   end Soul;
   type Soul_Ptr is access all Soul;
   procedure Free is new Ada.Unchecked_Deallocation (Soul, Soul_Ptr);

   type Abel is new Corpse with null record;
   function Get_Name (Context : Abel) return String;

   type Cain is new Corpse with null record;
   function Get_Name (Context : Cain) return String;
end Persons;
----------------------
with Ada.Text_IO;

package body Persons is   
   task body Soul is
   begin
      loop
         select
            accept Who_Are_You;
               Ada.Text_IO.Put_Line ("I'm " & Get_Name (Context.all));
         or terminate;
         end select;
      end loop;
   end Soul;
   
   function Get_Name (Context : Abel) return String is
   begin
      return "Abel";
   end Get_Name;

   function Get_Name (Context : Cain) return String is
   begin
      return "Cain";
   end Get_Name;
end Persons;
------------------------
with Persons; use Persons;
procedure Test is
   X : aliased Abel;
   Y : aliased Cain;
   Ptr : Soul_Ptr;
begin
   Ptr := new Soul (X'Unchecked_Access);
   Ptr.Who_Are_You;
   Free (Ptr);
   Ptr := new Soul (Y'Unchecked_Access);
   Ptr.Who_Are_You;
   Free (Ptr);
   delay 1.0; -- Let Text_IO task flush all buffers
end Test;

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