Path: utzoo!attcan!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: Multiple inheritance with virtual bases
Message-ID: <488@taumet.com>
Date: 22 Oct 90 15:00:34 GMT
References: <1990Oct21.211143.3459@nowhere.uucp>
Organization: Taumetric Corporation, San Diego
Lines: 30

sking@nowhere.uucp (Steven King) writes:

|In article |HAGIWARA.90Oct18105013@rd14.zuken.co.jp> hagiwara@zuken.co.jp (Kazuyuki Hagiwara) writes: 
|>
[ ... ]
|>struct A {
|>	char		avar0[1000];
|>};
|>struct B : virtual public A {
|>	int		bvar;
|>};
|>struct C : virtual public A {
|>	int		cvar;
|>};
|>struct D : virtual public B, virtual public C {
|>	int		dvar;
|>};

|	The definitions of B and C each have space for A allocated within them;
|thus the size of D is sizeof B ( which is sizeof A + pointer overhead ) +
|sizeof C ( sizeof A + pointer overhead ) + more pointer overhead. 

This is not correct.  Exactly one instance of a virtual base class exists
in an object by definition.  Class A is declared to be virtual in all uses
of it, so it is an error for there to be more than one copy.  Let us hope
that the error in this case is in the allocation, and that only one copy
is ever referred to by generated code.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com