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From: Duncan Sands <baldrick@free.fr>
Newsgroups: comp.lang.ada
Subject: Re: Return_By_Reference or Return_By_Copy (GNAT bug?)
Date: Sat, 1 Jan 2005 23:00:56 +0100
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Date: 2005-01-01T23:00:56+01:00

Hi Randy,

> A_Type certainly is return-by-reference.

thanks for confirming.

>     function Get_an_A return access A_Type;
> 
> Note that this makes the reference return explicit (no magical rules to page
> through).

I was playing around with return by reference to see if it is possible to
get unserialized access to a protected variable using the Rosen trick (it is,
see example below, especially the protected object P in package C).  Will this
be illegal in Ada 2005?

All the best,

Duncan.


package B is

   type A_Type is limited private;

   function Get_The_A return A_Type;

   procedure Increment (An_A : A_Type);

   procedure Print (An_A : A_Type);

private

   type R_Type (An_A : access A_Type) is limited null record;

   type A_Type is record
      R : R_Type (A_Type'Access);
      I : Integer := 0;
   end record;

end;

with Ada.Text_IO; use Ada.Text_IO;

package body B is

   The_A : A_Type;

   function Get_The_A return A_Type is
   begin
      return The_A;
   end;

   procedure Increment (An_A : A_Type) is
   begin
      An_A.R.An_A.I := An_A.R.An_A.I + 1;
   end;

   procedure Print (An_A : A_Type) is
   begin
      Put_Line (An_A.I'Img);
   end;

end;

with B; use B;

package C is

   protected P is

      function Get_A return A_Type;

   private

      P_A : A_Type;

   end P;

end;

package body C is

   protected body P is

      function Get_A return A_Type is
      begin
         return P_A;
      end Get_A;

   end P;

end;

with Ada.Text_IO; use Ada.Text_IO;
with B; use B;
with C; use C;

procedure Y is

   procedure Look is
      The_A_Reference : A_Type renames P.Get_A;
   begin
      Print (The_A_Reference);
      Increment (The_A_Reference);
      Print (The_A_Reference);
   end;

begin
   Put_Line ("Mechanism:" & Integer'Image (Get_The_A'Mechanism_Code));
   New_Line;
   Look;
   New_Line;
   Look;
end;