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

From: David Abrahams (dave_at_[hidden])
Date: 2008-08-25 11:51:02

on Mon Aug 25 2008, "Niels Dekker - mail address until 2008-12-31" <nd_mail_address_valid_until_2008-12-31-AT-xs4all.nl> wrote:

> David Abrahams wrote:

>>> 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.

What can I say; it's the best we can do.

>> 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?

That's a massive pessimization for fundamental types and simple
composites: int, T*, pairs, tuples, std::complex, etc... not to mention
that has_nothrow_copy *has* to give false negatives, even on a smart
compiler.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com