Question on virtual inheritance

[ambarish.mitra]

#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).

[Victor Bazarov]

am************@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
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[James Kanze]

On Apr 24, 3:58 pm, "Victor Bazarov" <[e-mail]:

[e-mail] wrote:

[...]

A pointer to the virtual base class subojbect[s] is usually stored
in the class (just like a pointer to the virtual function table),

There are at least two different solutions: CFront stored a
pointer to the virtual base in the class, but other
implementations put the information necessary to do the fix-up
in the vtable. It's also possible to use thunks to do any
fixing up needed; these would also be in the vtable.

Beyond that, there are a number of variations possible in the
details, and as you said, the only way to find out is to ask in
a forum dedicated to the compiler in question, or to look it up
in the compiler's documentation.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]