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

[joel falcou <joel.falcou@xxxxxx>]

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



--
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : (+33)1 69 15 66 35