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: 2008-08-25 08:04:04

David Abrahams wrote:
>> http://svn.boost.org/svn/boost/sandbox/move/boost/move/detail/has_swap_overload.hpp
> 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 
<boost/functional/detail/container_fwd.hpp>.

>> 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 
copy-construction?

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

Kind regards, Niels