Path: utzoo!attcan!uunet!cs.utexas.edu!ut-emx!nowhere!sking
From: sking@nowhere.uucp (Steven King)
Newsgroups: comp.lang.c++
Subject: Re: Multiple inheritance with virtual bases
Message-ID: <1990Oct21.211143.3459@nowhere.uucp>
Date: 21 Oct 90 21:11:43 GMT
Organization: nowhere
Lines: 41


In article <HAGIWARA.90Oct18105013@rd14.zuken.co.jp> hagiwara@zuken.co.jp (Kazuyuki Hagiwara) writes: 
>
>I am using AT&T C++ version 2.0.  Following is the simple test of
>multipile inheritance with virtual base classes. Do anyone know
>the reason why sizeof(D) is so large ?  I think sizeof(D) should be
>only 1000 bytes and some pointer overhead.
>
>----------------------------- mibug.C  {  ----------------
>#include <iostream.h>
>
>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. 

	A little thought on how virtual bases are implemented should reveal 
why this is, tho' I'm not sure that a smarter Cfront might only need to alloc
space for it once. 


-- 
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