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From: "Rego, P." <pvrego@gmail.com>
Newsgroups: comp.lang.ada
Subject: Constructors with multiple inheritance
Date: Thu, 29 Sep 2011 12:40:28 -0700 (PDT)
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Date: 2011-09-29T12:40:28-07:00
List-Id: <comp.lang.ada>

Hi people

I have a package which implements a class with two items which actually are=
 classes which builds different kind of queues. One of them is a queue whos=
e items are of the kind of the parent class. I need to implement a construc=
tor to the parent class, then I need to implement the constructors for the =
two queues. For me, it`s more intuitive from OO point of view that the thre=
e constructors are implemented as methods (not just independent calls), and=
 that was just the point where I got stuck.

So, the packages are declared as=20

------------------------
-- parent_package.ads --
------------------------
limited with Parent_Package.Child_Package;
package Parent_Package is
   type Parent_Class is tagged
      record
         Child : access Child_Package.Parent_Queue_Class;
         Modes : access Child_Package.Modes_Queue_Class;
      end record;
   type Parent_Class_Ptr is access all Parent_Class;
   function Construct (Sender : in Parent_Class) return Parent_Class_Ptr;
end Parent_Package;

------------------------
-- parent_package.adb --
------------------------
with Parent_Package.Child_Package;
package body Parent_Package is
   function Construct (Sender : in Parent_Class) return Parent_Class_Ptr is
      New_Parent : constant Parent_Class_Ptr :=3D new Parent_Class;
   begin
      New_Parent.Child :=3D Sender.Child.Construct;
      New_Parent.Modes :=3D Sender.Modes.Construct;
      return New_Parent;
   end Construct;
end Parent_Package;

--------------------------------------
-- parent_package-child_package.ads --
--------------------------------------
package Parent_Package.Child_Package is
   type Parent_Queue_Class is tagged
      record
         Next   : access Parent_Queue_Class;
         Parent : access Parent_Class;
      end record;
   type Parent_Queue_Class_Ptr is access all Parent_Queue_Class;
   function Construct (Sender : in Parent_Queue_Class) return Parent_Queue_=
Class_Ptr;

   type Modes_Queue_Class is tagged
      record
         Next : access Modes_Queue_Class;
         Mode : Integer;
      end record;
   type Modes_Queue_Class_Ptr is access Modes_Queue_Class;
   function Construct (Sender : in Modes_Queue_Class) return Modes_Queue_Cl=
ass_Ptr;
end Parent_Package.Child_Package;

--------------------------------------
-- parent_package-child_package.ads --
--------------------------------------
package body Parent_Package.Child_Package is
   function Construct
     (Sender : in Parent_Queue_Class) return Parent_Queue_Class_Ptr is
      pragma Unreferenced (Sender);
      New_Queue : constant Parent_Queue_Class_Ptr :=3D new Parent_Queue_Cla=
ss;
   begin
      return New_Queue;
   end Construct;

   function Construct
     (Sender : in Modes_Queue_Class) return Modes_Queue_Class_Ptr is
      pragma Unreferenced (Sender);
      New_Queue : constant Modes_Queue_Class_Ptr :=3D new Modes_Queue_Class=
;
   begin
      New_Queue.Mode :=3D 0;
      return New_Queue;
   end Construct;
end Parent_Package.Child_Package;

---------------------
-- parent_main.adb --
---------------------
with Parent_Package;
procedure Parent_Main is
   Obj : Parent_Package.Parent_Class_Ptr;
begin
   Obj :=3D Obj.Construct;
end Parent_Main;

---------------------------
when I build the code, I get the messages:
	warning: useless assignment to "Obj", value never referenced (ok, I agree =
sure)
	warning: null value not allowed here
	warning: "Constraint_Error" will be raised at run time
=09
and sure when I execute the code I get=20
	raised CONSTRAINT_ERROR : parent_main.adb:5 access check failed

	(BTW, if I implement the constructors independent from the class - i.e. wi=
thout referencing the parent class as the argument -, they return no error =
message and the results after that are according to expected, but sure it b=
reaks the oo design.)

So, how can I fix this?
Thanks!