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From: chaloux@maestro.mitre.org (Dave Chaloux)
Newsgroups: comp.lang.c++
Subject: <None>
Message-ID: <1991May28.162856.12685@linus.mitre.org>
Date: 28 May 91 16:28:56 GMT
Sender: news@linus.mitre.org (News Service)
Reply-To: chaloux@maestro.mitre.org (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
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I am new to C++ and have come across the following problem. Say that you
are building a communications program and you have two different kinds of
ports you must read and two kinds of messages that are naturally associated
with each port. In otherwords you have a Port_A class and a Port_B class.
They read and write respectively Message_A class objects and Message_B class
objects. Now say that you want to write your program such that you can
write a message to a port without knowing the kind of port or the kind of
message. No problem. Have a base Message class and a base Port class. Have
virtual functions for the Port class that write Message_A and Message_B type
objects. Now have a write virtual function for class Message that takes as
an argument an object of type Port. Now we might write the message as
follows:

message->write(&port_object)

Since a class derived from a parent class can be used in place of the parent
class, it doesn't matter whether the port_object is a Port_A object or a 
Port_B object. The appropriate function for either Port_A or Port_B gets called
depending on whether the message was a Message_A or Message_B message. The
message gets written and everyone is happy, EXCEPT THE COMPILER.
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?

This problem must have come up before. Is there a standard way for solving it?
The only method I can figure out is to have two different header files for one
of the classes. The one file has no function definitions in it. Therefore it does
not need to include the header file for the other class. This header file can then
be included by the other class without things breaking.

Thanks in advance.