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From: jbuck@forney.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: For Philosophers and Lawyers
Message-ID: <1991May24.012020.9980@agate.berkeley.edu>
Date: 24 May 91 01:20:20 GMT
References: <519f23cc.19260@apollo.HP.COM> <May.17.17.47.20.1991.1598@morley.rutgers.edu> <72535@microsoft.UUCP>
Sender: root@agate.berkeley.edu (Charlie Root)
Reply-To: jbuck@forney.berkeley.edu (Joe Buck)
Organization: University of California, Berkeley
Lines: 30

This thread addresses problems with casts between pointer types that
are declared (i.e. "class A;") but not defined, and the problems
that arise with multiple inheritance.

Jim Adcock points out that
>C++ allows these unsafe casts so that programmers can use pointers to
>objects without always having to include class declarations.

This is a very strong argument; those of us that want our large C++
projects to compile in reasonable time rely on incomplete class
declarations.  Also, E&S discusses this very issue on pp. 68-69.
They give the example

class S;
class T;

T* f(S* p) { return (T*)p;}

class A { int a;};
class S { int s;};
class T : public A, public S { int t; };

This code will do The Wrong Thing.  Their conclusion?

"It follows that where multiple inheritance is involved casting to
and from undefined types is best avoided."

--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck