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

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Yes, this is the problem, but I can't make this copy constructor
private, because it is being used when a Map expression gets nested
into a bigger expression...

Right now I am looking for a minimal-effort solution as there are more
pressing things to do...

Benoit

2010/4/5 Eamon Nerbonne <eamon.nerbonne@xxxxxxxxx>:
> Isn't the issue really that Eigen::Map isn't really a value?  Eigen::Map is
> more like a pointer, except it's used like a value, syntactically
> I'm not sure this is actually a good idea, but an alternative would thus be
> to split Eigen::Map into a "pointer" and a "value" type so that you can swap
> the pointers as expected and prevent swapping on values by disabling the
> copy-constructor (you can't really copy-construct those anyhow - which leads
> to this problem).
> Basically, Eigen::Map's copy constructor isn't really behaving like you'd
> expect a copy constructor to behave.  Which isn't a big issue, granted ;-).
> --eamon@xxxxxxxxxxxx - Tel#:+31-6-15142163
>
>
> On Mon, Apr 5, 2010 at 23:55, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>>
>> Hi Joel,
>>
>> 2010/4/5 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.
>>
>> We already have a swap() that works well in Eigen, depending on two
>> template parameters. It has a different calling syntax: it is a member
>> function. a.swap(b).
>>
>> Here the problem is with users who expect std::swap to work on the Map
>> expression (where it is really tricky), and discover that it doesn't.
>> If we decide that it's OK to tell them to use a different swap()
>> function, then Eigen's current solution is already good enough.
>>
>> I was wondering if we could do something to prevent the bad surprise
>> with std::swap from happening in the first place. I'll try to get a
>> static assertion to work, with an informative error message.
>>
>> Benoit
>>
>> >
>> > 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
>> >
>> >
>> >
>> >
>> >
>>
>>
>
>