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From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-08-25 08:04:04

David Abrahams wrote:
> I think that might be an older version of the same functionality. In
> fact, I think I forgot about the old version when I posted the one I
> did. So you might compare and see if one is better.

It's actually the other way around: The version that Daniel James added
to svn/boost/sandbox/move is the better one. It has three more
has_swap_overload specializations: for std::basic_string, std::multiset,
and std::multimap. And it has the std forward declarations included from

>> It's a pity that has_swap_overload<T> yields false negatives, when T
>> has
>> std as associated namespace.
> It's easy enough to fix that with specialization, as my posted example
> shows.

Hmmm... So has_swap_overload<T> should have a specialization for each
class template that has a template type parameter (allowing std as
associated namespace of the template argument) and a custom swap. That's
a pity.

> IMO the best solution is like this:
> std is an associated namespace of T
> ? has_member_swap<T>
> : has_nonmember_swap<T>
> plus any specializations required to handle something like tr1::array
> that might not have a member swap.

FYI, the tr1::array that comes with VC9 SP1 /does/ have a member swap,
as well as a swap overload.

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

  if tr1::has_nothrow_copy<T>
    move by means of copy
    move by means of default-construction + swap

Kind regards, Niels

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