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

From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-08-25 11:34:57

Niels Dekker - mail address until 2008-12-31 wrote:
> 
> 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
> 

Unless you have a type like boost::function apparently is currently 
where default construction + swap is also not a no throw operation and 
is more expensive than a direct copy.

Unless you consider swaps like boost::function's a bug I don't think it 
is fair to assume swap means fast move in general. I think the only real 
way to get it right in every case is to have the author of the type 
specify the proper traits.