Re: Linux vs. libio

[Jamie Lokier <jamie dot lokier at cern dot ch>]

Mark Mitchell wrote:
> With the new ABI, pointers to virtual tables come at a different point
> in the objects.  Bases are laid out in different orders.  There are no
> longer pointers to virtual bases.  Some of that doesn't matter; some
> of it does.  Given:
> 
>   struct _ios_fields { ... };
>   class ios : public _ios_fields { ... };
> 
> it is no longer the case that you can cast an `ios *' to an
> `_ios_fields *' without having to do modify the pointer value;

> More importantly, the same holds for streambuf and _IO_FILE.  You
> can't cast an _IO_FILE to a streambuf, or vice versa, without
> modifying the pointer value.

<evil>
You might be able to do it by reserving a field in the new structures at
the right place to contain the old-Glibc-compatible vtable pointer, and
having two separate vtables.  Only one vtable (and corresponding
structure layout) is active at a time.  The other points to a set of
thunks.  When you call a thunk, it modifies the vtable pointers to swap
which is the active vtable, reorders the data fields for the other ABI
and continues from there.  
</evil>

;-)

-- Jamie