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From: Mirko Aigner <Mirko.Aigner@alcatel.de>
Newsgroups: comp.lang.ada
Subject: Re: circular unit dependency
Date: Mon, 26 May 2003 10:05:28 +0200
Organization: Alcatel SEL AG Berlin
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Date: 2003-05-26T10:05:28+02:00
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Stephen Leake wrote:
> 
> You will have to change something. You'll have to post the code, so we
> can help you figure out what to change. Try to simplify it first.
> 

I'm Sorry, Ok here comes some code.

Some words to the structure at first. There is a boundary subsystem 
where logical Messages are generated. These Messages can be from 
different type (class). All Messages are sent first to a dispatcher in 
the Control Subsystem. The  dispatcher send them to subscribed 
Control-Classes. The Control-Classes react to the messages.
The Control-Classes can be dynamically subscribed or unsubscribed to the 
dispatcher. Now there is used a design pattern called Visitor, so that 
the dispatcher can decide what kind of message he got.

I can't change the design, i have to implement it in Ada. It is already 
imlemented in C++.

here is some code ...

-- Event.ads

with Class_EventVisitor; use Class_EventVisitor;

package Class_Event is

   type Event is tagged null record;
   type Event_Pointer is access all Event'Class;

   procedure acceptVisitor( this : in Event_Pointer; visitor : in 
EventVisitor_Pointer ) is abstract;

end Class_Event;



-- Event.Message

with Class_EventVisitor; use Class_EventVisitor;

package Class_Event.Class_Message is

   type Message is new Event with null record;
   type Message_Pointer is access all Message'Class;

   procedure acceptVisitor( this : in Message_Pointer;
                            visitor : in EventVisitor_Pointer );

end Class_Event.Class_Message;


-- Event.Message.PointMessage

with Class_EventVisitor; use Class_EventVisitor;

package Class_Event.Class_Message.Class_PointMessage is

   type PointMessage is new Message with private;
   type PointMessage_Pointer is access all PointMessage'Class;

   procedure acceptVisitor( this : in PointMessage_Pointer;
                            visitor : in EventVisitor_Pointer );

   function Create ... return PointMessage;

private
   type PointMessage is new Message with record
      ...
   end record;

end Class_Event.Class_Message.Class_PointMessage;


-- EventVisitor

with Class_Event.Class_Message.Class_PointMessage;
use Class_Event.Class_Message.Class_PointMessage;

package Class_EventVisitor is

   type EventVisitor is tagged private;

   type EventVisitor_Pointer is access all EventVisitor'Class;

   procedure handlePointMessage ( this : in EventVisitor_Pointer;
                                  pMessage : in PointMessage_Pointer )
                                  is abstract;

   -- other message types
   ...


private
   type EventVisitor is tagged null record;

end Class_EventVisitor;

The Control-Class handling the PointMessage will inherit from the 
EventVisitor.