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From: dog@cbnewsl.ATT.COM (edward.n.schiebel)
Newsgroups: comp.lang.c++
Subject: Re: A solution to the multiple inclusion problem
Message-ID: <2466@cbnewsl.ATT.COM>
Date: 27 Oct 89 11:09:51 GMT
References: <1030@ncratl2.Atlanta.NCR.COM>
Distribution: comp.lang.c++
Organization: AT&T Bell Laboratories
Lines: 32

From article <1030@ncratl2.Atlanta.NCR.COM>, by dspoon@ncratl2.Atlanta.NCR.COM (Dave Witherspoon):
> [...stuff deleted]
> 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):
> 
> A.hpp                                B.hpp
> -----                                -----
> #ifndef D_A                          #ifndef D_B
> #define D_A                          #define D_B
> #include "B.hpp"                     #include "A.hpp"
> 
> class A { B* bp;}                    class B {A* ap;}
> #endif                               #endif
> 
Since class A only has a B*, and not a B you don't need to #include B.hpp
but only tell the compiler B is a class. Thus:

#ifndef D_A                          #ifndef D_B
#define D_A                          #define D_B

class B;                             class A;	// DON'T NEED INCLUDE!
 
class A { B* bp;}                    class B {A* ap;}
#endif                               #endif

Now, if A included an instance of a B and visa-versa, well, then
you have problems.

	Ed Schiebel
	AT&T Bell Laboratories
	dog@vilya.att.com
	201-386-3416