C++ vtable example

[Jason Merrill]

First, I'd like to propose again that the RTTI info move to negative
offsets, for COM compatibility.

I believe your D vtable is wrong; for starters, there's only one A vtable
in D.  Also, calls through defining class vtables don't require adjustment;
you seem to have that backwards.  My interpretation of Christophe's
proposal for this case:

D::offset_to_A (32)
D::offset_to_top (0)
D::rtti
-- D, B-in-D vtable address --
B::f() [0]

C::offset_to_A (16)
C::offset_to_top (-16)
C::rtti
-- C-in-D vtable address --
C::g() [0]

A::offset_to_top (-32)
A::rtti
-- A-in-D vtable address --
B::f() [offset at -80] -- change from A-in-B vtable, requires third-party thunk
C::g() [offset at -48]

-----------

For comparison, Brian's and my proposal would look like this:

D::offset_to_A (32)
D::offset_to_top (0)
D::rtti
-- D, B-in-D vtable address --
B::f() [0]

C::offset_to_A (16)
C::offset_to_top (-16)
C::rtti
-- C-in-D vtable address --
C::g() [0]

A::offset_for_g (-16)
A::offset_for_f (-32)
A::offset_to_top (-32)
A::rtti
-- A-in-D vtable address --
B::f() [offset at -24]
C::g() [offset at -32]