[Bug ada/34504] New: Legal program runs incorrectly, dynamic dispatching with interfaces, RM 3.9.2(5/2, 20/2), 3.9.4(1/2)

["ludovic at ludovic-brenta dot org" <gcc-bugzilla at gcc dot gnu dot org>]

--   Newsgroups: comp.lang.ada
--   From: "Peter C. Chapin" <pcha...@sover.net>
--   Date: Thu, 13 Dec 2007 07:20:46 -0500
--   Subject: Q about finalization and interfaces.
--
--   I'm trying to understand the interaction between controlled types,
--   interfaces, and class-wide dispatching. Accordingly I wrote the
--   following program, which I will present in sections. First the package
--   specification:

   with Ada.Finalization;
   package Check_Package is

      type B is interface;
      procedure Do_Stuff( Thing : in B ) is abstract;

      type D is new Ada.Finalization.Controlled and B with
         record
            X : Integer := 0;
         end record;

      overriding procedure Do_Stuff( Thing : in D );
      overriding procedure Finalize( Thing : in out D );

   end Check_Package;

--   The idea is that I want to build a derivation class rooted on the B
--   interface with some of the types in that class (such as D) being
--   controlled. The corresponding body prints out a few messages to help
--   track what is happening. Note that I use the X component as a kind of
--   object ID number.

   with Ada.Text_IO; use Ada.Text_IO;
   with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
   package body Check_Package is

      procedure Do_Stuff( Thing : in D ) is
      begin
         Put("Do_Stuff( Thing : in D ) => ");
         Put(Thing.X);
         New_Line;
      end Do_Stuff;

      procedure Finalize( Thing : in out D ) is
      begin
         Put("Finalize( Thing : in out D ) => ");
         Put(Thing.X);
         New_Line;
      end Finalize;

   end Check_Package;

--   Now the test program that exercises the above code looks like this:

   with Check_Package; use Check_Package;
   procedure Check is
      Object     : D;
      Some_Thing : B'Class := Object;
   begin
      Object.X := -1;
      Do_Stuff(Some_Thing);
   end Check;

--   Using GNAT GPL 2007 I get the following output:
--
--   Finalize( Thing : in out D ) =>           0
--   Finalize( Thing : in out D ) =>          -1
--
--   My interpretation is that Object is copied (with the default ID value
--   of zero) when Some_Thing is initialized; Object's ID is then modified.
--   Both Object and its copy are finalized when the program ends. But...
--   what happened to my call to Do_Stuff?? I expected that call to dispatch
--   on Some_Thing and invoke the Do_Stuff for type D. What am I
--   misunderstanding?
--
--   Thanks in advance!
--
--   Peter


-- 
           Summary: Legal program runs incorrectly, dynamic dispatching with
                    interfaces, RM 3.9.2(5/2, 20/2), 3.9.4(1/2)
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ludovic at ludovic-brenta dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34504