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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Task components, the rationale
Date: Wed, 13 Jul 2011 20:52:02 +0200
Organization: cbb software GmbH
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Date: 2011-07-13T20:52:02+02:00
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OK, to avoid false impression that Ada was carelessly designed, it must be
said that there was a valid reason why task components are broken this way.
That is to prevent a much worse disaster when so-called Rosen's trick is
used.

Consider the pattern discussed earlier (the Rosen's trick):

   type T;
   task type Worker (Self : not null access T'Class);
   type T is new Ada.Finalization.Limited_Controlled with record
      My_Worker : Worker (T'Access);
   end record;
   overriding procedure Initialize (Object : in out T);
   procedure Foo (Object : in out T) is abstract; -- A primitive operation
   
Now, if My_Worker started before completion of Initialize then this body

   task body Worker is
   begin
       Self.Foo; -- Boom!

could call Foo of T or any of its derived type *before* Initialize, i.e.
before the object's construction is done! That would be a much worse
problem.

There is no simple solution for this. To start with tasks must be
inheritable from and their bodies must be primitive or class-wide
operations, because aggregation (composition) + Rosen's trick is
necessarily broken.

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