From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-08-25 11:34:57
Niels Dekker - mail address until 2008-12-31 wrote:
>
> Anyway, I was also considering a more "lightweight" way to tweak MSVC's
> Swaptimization, merely based upon tr1::has_nothrow_copy (which is
> included VC9 SP1). Because when using MSVC9, whenever a function like
> vector<T>::push_back needs to reallocate, it moves its elements /either/
> by copy-construction /or/ by default-construction + swap, depending on
> whether "Swaptimization" is turned on for type T. Isn't it reasonable
> to just do default-construction + swap, /unless/ T has no-throw
> copy-construction?
>
> if tr1::has_nothrow_copy<T>
> move by means of copy
> else
> move by means of default-construction + swap
>
Unless you have a type like boost::function apparently is currently
where default construction + swap is also not a no throw operation and
is more expensive than a direct copy.
Unless you consider swaps like boost::function's a bug I don't think it
is fair to assume swap means fast move in general. I think the only real
way to get it right in every case is to have the author of the type
specify the proper traits.