Vcall/vbase offset clarification

[Jim Dehnert]

> From: Martin von Loewis <loewis at informatik.hu-berlin.de>
> To: mark at codesourcery.com
> 
> > > From mitchell at codesourcery.com  Tue Jan 25 13:26:12 2000
> > >   struct A { void f();}
> > >   struct B : public virtual A {};
> [...]
> > >   o First, we convert to the most derived class in which `f' was
> > >     overridden.  That's `A'.  We know statically what offset
> > >     to adjust `b' by to get an `A*'.
> 
> Maybe you didn't mean it that way, but: How do we know statically the
> offset needed to adjust a B* to an A*? Wouldn't we have to look in the
> vtable at run time to fetch the vbase offset?
> 
> > >   o We pass this value to the function found in the vtable for `A'.
> > > 
> > >   o That function will first perform the constant adjustment from 
> > >     `A' to `B'.  
> 
> Or perhaps that shouldn't have been virtual inheritance in the B
> bases?

I think the right statement is 'we know statically how to adjust "b" to
get an "A*".  If A is a non-virtual base, we know the offset
statically.  If it is a virtual base, we know statically where to fetch
the vbase offset in the vtable.'

Jim

-	    Jim Dehnert		dehnert at sgi.com
				(650)933-4272