Re: [eigen] std::swap doesn't work on Map

[Manoj Rajagopalan <rmanoj@xxxxxxxxx>]

Shall I file a bug report so this stays on the radar? Or can this be included 
in a "Known Issues" section in the Eigen documentation? Using swap() in my 
program made it buggy and since Eigen was new, I took no chances and 
experimented with just about every construct I was using and discovered this 
behavior. Others can benefit by not having to rediscover this.

Thanks,
Manoj


On Monday 05 April 2010 01:31:19 am joel falcou wrote:
> Benoit Jacob wrote:
> > Normally we could consider fixing this by providing a partial template
> > specialization of std::swap for Eigen types, but the big problem is
> > that std::swap takes only 1 template parameter and expects both sides
> > to be of the same type, and this is not always the case with Eigen
> > swap on expressions. We could still catch the case where both sides
> > have the same type, but I'm not sure what to do then. If we let that
> > work, it's awkward that we require both sides to have the same type in
> > std::swap and not in .swap(). If we want to emit an error, it's hard
> > to do because of SFINAE (so generating an error there would simply
> > discard the specialization.). I have to read back the rules of SFINAE,
> > perhaps it will work to let our std::swap specialization call another
> > function from where we trigger an error...
> >
> > Benoit
>
> Look how boost::swap works, there is a simple trick to have your own
> eigen::swap that behaves correctly
> without tricky SFINAE. Basically :
>
> namespace eigen {
> namespace details
> {
>   template<class T1,class T2>
>   void swap( T1& a, T2& b)
>  {
>     using std::swap;
>     swap(a,b);
>  }
> }
>
> template<class T1,class T2>
> void swap( T1& a, T2& b)
> {
>    details::swap(a,b);
> }
>
> }
>
> the main swap having *two* templates arguments make it more specialized
> than the ADL found swap for type T1 and T2.
> Then the using clause in the details::swap bring std::swap in the game
> if and only if the proper swap can't be found by ADL.
>
> Using this swap then behaves like that:
> if a swap exists in the namespace of T1, it is used.
> else std::swap is used silently.
>
> Now, just tell user to use eigen::swap.
>
> Reference here:
>
> http://www.boost.org/doc/libs/1_42_0/boost/utility/swap.hpp