dolfin team mailing list archive

[Jed Brown <jed@xxxxxxxx>]

Matthew Knepley wrote:

> I believe that the cast causes it to offset into the object structure.
> Everyone still points to the same virtual function table

This example has no vtable, but if it did, both the struct and the
corresponding vtable would be concatenated from A and B.  C would have a
layout equivalent to the C struct:

struct C {
  void *vptr_A;
  int avar;
  void *vptr_B;
  int bvar;
};

with vtable

struct C_vtable {
  void (*amethod)();
  void (*cmethod)();
  void (*reserved)();       /* I don't know what this space is for */
  void (*typeinfo_for_C)();
  void (*bmethod)();
};

C::vptr_A points to the beginning of C_vtable while C::vptr_B points to
the offset.  (This is my recollection from vtable debugging a while ago,
I'm sure these layout details are somewhat gcc-specific, but I don't
think anyone does multiple inheritance substantially differently.)

> On Mon, Nov 9, 2009 at 7:12 AM, Anders Logg <logg@xxxxxxxxx
> <mailto:logg@xxxxxxxxx>> wrote:
> 
>     I'm struggling with some problems in the implementation of automatic
>     update/interpolation during mesh refinement.
> 
>     I've narrowed one of the problems down to the following simple
>     example. Say that a class C inherits from both A and B (which both
>     have member variables - this is important). Then I would expect an
>     object c of class C to have the same address/pointer as seen from all
>     three classes.

It could not possibly work this way because B must be able to function
correctly with no knowledge of A and C.


Jed

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