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