Pointers-to-members

[Mark Mitchell]

Following up on my own suggestion:

  The ABI document says that a NULL pointer-to-member function has 
  `ptr == 0'.  It does, not, however say whether or not a NULL
  pointer-to-member function also has `adj == 0'.

  I believe that this should be specified as well so that code generated
  to do comparison of pointers to members (of the same type) looks like:

    p1->ptr == p2->ptr && p1->adj == p2->adj

  and not:

    p1->ptr == p2->ptr && (!p1->ptr || (p1->adj == p2->adj))

  So, I would say:

    If the pointer-to-member is NULL, both fields are zero.  (Note:
    there are no non-NULL pointers-to-members for which the `ptr'
    field is non-zero.)

It's occurred to me that this imposes some overhead on casting
pointers-to-members around: now when you convert from a base pointer
to member to a derived version (or vice versa), you can't just adjust
the `adj' member willy-nilly; instead, you have to check first whether
or not the pointer is NULL.

So, I'm not sure any more which scheme is preferable -- but we
definitely need to say clearly which we want.

--
Mark Mitchell                   mark at codesourcery.com
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