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From: rmartin@clear.com (Bob Martin)
Newsgroups: comp.lang.c++
Subject: Re: <None>
Message-ID: <1991Jun6.144347.13450@clear.com>
Date: 6 Jun 91 14:43:47 GMT
References: <1991May28.162856.12685@linus.mitre.org>
Organization: Clear Communications, Inc.
Lines: 68

In article <1991May28.162856.12685@linus.mitre.org> chaloux@maestro.mitre.org (Dave Chaloux) writes:
... Paraphrase...
	[Dave has base class for ports and a base class for messages and wants
	all derived ports and messages to support the following semantics:]
>
>message->write(&port_object)

> [... stuff deleted...]

>To set this up, class Port needs to know about Message_A class and Message_B
>class. Message_A and Message_B class need to know about type Port.
>
>Now imagine you have your include files. The header for class Message_A includes
>Port.h . The header for Port has to include Message_A.h . When compiling, one
>of the class definitions is going to come first and the definition for the
>other class is undefined. OUCH.
>
>WHERE AM I MISSING THE BOAT?
>

Dave:

Try this:
	
	----------------  message.h ----------------
	class Port; // declaration of class port.. #include "port.h" not needed.

	class Message
	{
		...
		void write(Port&);	
		...
	};

	--------------- port.h ----------------------
	class Message; // declaration again.  #include "message.h" not needed

	class Port
	{
		...
		void write(Message&);
		...
	}

	----------------- message.cc --------------
	#include "message.h"
	#include "port.h" // message implementation needs to know port semantics.
	void Message::write(Port& thePort)
	{
		thePort.write(*this);
	}

	------------------ port.cc ----------------
	#include "port.h"
	void Port::write(Message& theMessage)
	{
		... whatever ports do.
	}

In general the following rule will help keep you out of trouble...
	If the specification of a class does not need to know the semantics of 
	one of its constituents, then the constituent class should be declared, 
	not #included. 
-- 
+-Robert C. Martin-----+:RRR:::CCC:M:::::M:| Nobody is responsible for |
| rmartin@clear.com    |:R::R:C::::M:M:M:M:| my words but me.  I want  |
| uunet!clrcom!rmartin |:RRR::C::::M::M::M:| all the credit, and all   |
+----------------------+:R::R::CCC:M:::::M:| the blame.  So there.     |