Re: [boost] Re: Reference counting 'any'

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-07-20 12:36:12

From: "Kevlin Henney" <kevlin_at_[hidden]>
> >   From: "Peter Dimov" <pdimov_at_[hidden]>
> >Why is a copy-on-write 'any' implementation non-conforming? What (single
> >threaded) valid user program does not work with a COW implementation?
>
> It is not that COW is non-conforming, it is that COW is problematic and
> often does not deliver the ROI expected of it.

One common situation where COW does deliver is when any's are placed in a
std::vector.

I think I understand why COW is problematic for std::string, but I've
difficulty envisioning the potential problems with a COWed 'any'. Could you
elaborate please?

> >[Note: the current spec does not specify the validity of the pointer
> >returned by any_cast. Therefore it should be interpreted to be valid
until
> >the next 'any' operation. I think that this can be strengthened to 'the
next
> >non-const operation' and a COW implementation would still conform.]
>
> I think this is a weakness and oversight in the current specification
> for 'any' (so thanks for pointing this one out!) rather than a loophole
> to be exploited.

A loophole? I think that guaranteeing validity until the first non-const
operation is quite natural; the pointer can't remain valid after operator=
or clear(). [Hmm, any doesn't have a clear(). :-) No matter, if it had one
it would invalidate.]

How would you specify the pointer validity?

--
Peter Dimov
Multi Media Ltd.