[RFC] C++1x breaking the ABI in one more place :(

[Mark Mitchell]

Gabriel Dos Reis wrote:

>> I do think the earlier suggestion of "add a few spare vtable slots" is a
>> good bit of future-proofing, in that it improves the odds that any given
>> change will be ABI-compatible.
> 
> How many "spare vtable slots" is future-proof? 12? 24? 1024? 1G?

Obviously, no number is sufficient in theory.  But, even two or three
would probably be sufficient in practice, and cost just a few bytes.  It
seems like a good call.

> Personally, I believe that the risk posed by the original patch is being
> inflated beyond the reasonable.

My view of this is that either you maintain ABI compatibility or you
don't.  Once you start guessing about what you can get away with, I
think you've already lost.

One of the reasons I feel that way is that to convince users you're
serious you have to an unambiguous answer.  Once you start saying "We
maintain compatibility, except ..." you lose credibility with your
audience, who often doesn't know whether it's codebase might or might
not be affected.  I think G++ has gotten a bad reputation around this
point, and that we can restore that reputation by being unambiguous:
unless we change the .so version, it's compatible.  Period.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713