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

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

hm, you cannot have partial specializations but you can still have overloads, and so the following works well here:

namespace std {
  template<typename A,typename B> void swap(A& a, B& b) { a..swap(b); }

};

And of course we can then rely on a small helper struct to leverage partial specialization if needed (e.g., to output a nicer compilation error if A and B are not Eigen objetcs etc.)

Maybe I'm missing something ?

gael

On Tue, Apr 6, 2010 at 1:03 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:

2010/4/6 joel falcou <joel.falcou@xxxxxx>:

> Benoit Jacob wrote:
>>
>> 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).
>>
>
> What I tell you is to have a eigen::swap(a,b) free function in eigen
> namespace
> and promote its use in place of std::swap ;)
>
> Then you will be able to specialize it properly AND let ADL do its tricks.

Ok, thanks for your suggestion, actually I think I had understood it,
but I am coming from a different perspective. I was wondering: can I
fix std::swap or do I have to tell the user to do something else? If I
have to tell him to do something else, telling him to do a.swap(b); is
as good, from my perspective, as telling him to do Eigen::swap(a,b);

I think I understand where Boost is coming from: if the namespace
boost is "used" instead of namespace std, then the correct swap is
automatically used, which is great.

But in the case of Eigen, I want it to remain possible to do both:
using namespace std;
using namespace Eigen;

which means that I can't define a global swap() function, as it would
conflict  with std::swap ! So I would have to call it ei_swap which
would defeat the point (the ability to switch swap functions just by
using a different namespace).

I guess this is where Eigen is different from Boost and can't do the same.

Cheers,
Benoit

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