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From: shap@delrey.sgi.com (Jonathan Shapiro)
Newsgroups: comp.lang.c++
Subject: Re: A solution to the multiple inclusion problem
Message-ID: <1144@odin.SGI.COM>
Date: 27 Oct 89 17:31:48 GMT
References: <14240@well.UUCP> <1989Oct23.191634.6345@cs.rochester.edu> <950@dutrun.UUCP> <1030@ncratl2.Atlanta.NCR.COM>
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Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <1030@ncratl2.Atlanta.NCR.COM> dspoon@ncratl2.Atlanta.NCR.COM (Dave Witherspoon) writes:
>
>Say I have 2 classes, A and B.  A contains a B*, and B contains an A*.
>Thus, we have a circular dependency!  One proposal (that I've tried):
>

The following should do what you want.  The trick is to recognize that
you don't really have a circular dependency at all:

A.hpp                           B.hpp
-----                           -----

#ifndef A_HPP                   #ifndef B_HPP
#define A_HPP                   #define B_HPP

class B;                        class A;

class A { B* bp; };             class B {A* ap;};


All C++ needs to know is that A/B are classes to construct a pointer
to them.  It doesn't need to know what the contents are until you go
to use them.

The only disadvantage to this approach is that A's inline member
functions won't get inlined into the B inline member functions that
are defined in the header, and vice versa.

This sort of problem is tricky.  If you try to do what you were doing,
it is very easy for a 60 line implementation file to haul in 6000
lines wirth of headers.  I've seen it happen.