Re: [boost] Enabling fast swap optimization for Boost types VC9 SP1.

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-08-27 12:13:04

David Abrahams wrote:
> But can you come up with a type where it makes sense to have
> written a custom swap but default-construction + swap won't
> outperform copy construction?

What do you think of the ticket I recently submitted, requesting custom 
swap functions for value_initialized<T>? 
http://svn.boost.org/trac/boost/ticket/2243  After having added 
value_initialized<T>::swap according to the ticket, it would very much 
depends upon T whether or not default-construction + swap would 
outperform copy-construction, for value_initialized<T>.

>> Do you know how we can technically overrule the default
>> _Move_operation_category<T> provided by MSVC's STL?
>
> If they didn't give us any further hooks (such as a default argument)
> to hang a specialization on, then sadly I don't know of a way.

Their macro _DEFAULT_MOVE_OPERATION_CATEGORY looks a little bit like a 
hook, but it wasn't intended that way, as Stephan T. Lavavej told me at 
http://blogs.msdn.com/vcblog/archive/2008/08/11/tr1-fixes-in-vc9-sp1.aspx

A related issue: the trunk version of Boost Optional now allows 
boost::optional<T>::swap to be tweaked in a similar way, following a 
request by Thorsten, here at [boost]. Optional's swap function uses 
default-construction + swap when specified by 
boost::optional_swap_should_use_default_constructor<T>: 
http://svn.boost.org/svn/boost/trunk/boost/optional/optional.hpp  I 
think it would be preferable to have Optional's swap tweaking based upon 
the same type traits that activate "Swaptimization" on MSVC's STL.

Kind regards, Niels