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From: "Nick Roberts" <Nick.Roberts@dial.pipex.com>
Subject: Abstract Completion Question
Date: 1999/03/04
Message-ID: <7bmjrp$5u5$1@plug.news.pipex.net>#1/1
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Newsgroups: comp.lang.ada
Date: 1999-03-04T00:00:00+00:00
List-Id: <comp.lang.ada>

-- Consider the following code:

with Ada.Text_IO; use Ada.Text_IO;

procedure Test_1 is

   package P1 is
      type T1 is abstract tagged null record;
      procedure A (X: in T1) is abstract;
   end;

   type T2 is new P1.T1 with
      record
         Data: String(1..10) := "***DATA***";
      end record;

   procedure A (X: in T2) is
   begin
      Put_Line(X.Data);
   end;

   type T1CA is access all P1.T1'Class;

   Y: aliased T2;
   Z: T1CA := Y'Access;

begin

   P1.A(Z.all); -- dispatching call

end;

-- The operation "A" of type "T2" is overridden, so fulfilling the promise
-- of type "T1" (as far as Ada is concerned).  This program compiles
-- and runs (GNAT 3.11p Win32), but nevertheless "A" is not a primitive
-- operation of "T2" (because it is not declared in a package spec), so
-- "A" cannot be dispatched to.  Upon executing this program under
-- GNAT, Constraint_Error is raised.  This behaviour could be a
-- 'surprise' for users (a nasty one).  Comments?

-------------------------------------
-- Nick Roberts
-------------------------------------