Re: [eigen] min(Arr1,Arr2) is it supposed to work?

[Karl Rupp <rupp@xxxxxxxxxxxxxxxx>]

Hi guys,

I just stumbled over a similar issue in a totally different context 
yesterday - still, it might be just what is needed: What about using an 
enable_if variant for the overloads? Something like

template <typename LHS, typename RHS>
typename enable_if<is_eigen<LHS>::val || is_eigen<RHS>::val,
                   ReturnTypeHere>::type
min(LHS const & lhs, RHS const & rhs) { ... }

The metafunction is_eigen<> should be such that it returns true if the 
argument is an Eigen type, otherwise false. In the body of min, you can 
then call e.g. Eigen::min(const ArrayBase<T>&, const ArrayBase<T>&)

Details on enable_if can be found e.g. here:
http://www.boost.org/doc/libs/1_46_1/libs/utility/enable_if.html
The implementation of enable_if is trivial, so no need to worry about 
boost dependencies.

Maybe that helps...

Best regards,
Karli


On 04/04/2011 04:23 PM, Gael Guennebaud wrote:
> oops, indeed the quick ref guide is again in advance with Eigen
> development. Actually, it seems to be impossible for us to support
> std::min(array1,array2) because std::min is implemented in a generic
> way as:
>
>      template<typename _Tp>   inline const _Tp&   min(const _Tp&  __a,
> const _Tp&  __b)
>      {
>        if (__b<  __a)
> 	return __b;
>        return __a;
>      }
>
> therefore we cannot overload std::min for min(const
> ArrayBase<T>&,const ArrayBase<T>&). To do so we would need overloads
> for every expressions (Array<>, CwiseUnaryOp, CwiseBinaryOp, etc.).
>
>
> I'm fixing the doc.
>
> gael
>
>
> On Fri, Apr 1, 2011 at 9:23 AM, Helmut Jarausch
> <jarausch@xxxxxxxxxxxxxxxxxxx>  wrote:
> > Hi,
> >
> > is the following assumed to work (I saw it in the Quick Reference
> > Guide)
> >
> > #include<Eigen/Core>
> > #include<cmath>
> > given
> > Eigen::Array3f V1, V2, V3;
> >   V1<<  1,2,3;
> >   V2<<  2,1,5;
> >   V3= V1.min(V2);       // this works
> >   V3= std::pow(V1,2);   // this works, as well
> >
> >   V3= std::min(V1,V2);  // but this fails, see below
> >   cout<<  V3<<  endl;
> > }
> >
> > Tmin.C:14:21:   instantiated from here
> > /usr/lib/gcc/x86_64-pc-linux-gnu/4.5.2/include/g++-v4/bits/
> > stl_algobase.h:191:7: error: could not convert '((const
> > Eigen::ArrayBase<Eigen::Array<float, 3, 1>  >*)((const
> > Eigen::Array<float, 3, 1>*)__b))->Eigen::ArrayBase<Derived>::operator<
> > [with OtherDerived = Eigen::Array<float, 3, 1>, Derived =
> > Eigen::Array<float, 3, 1>, typename Eigen::internal::traits<T>::Scalar
> > = float](((const Eigen::ArrayBase<Eigen::Array<float, 3, 1>  >&)((const
> > Eigen::ArrayBase<Eigen::Array<float, 3, 1>  >*)((const
> > Eigen::Array<float, 3, 1>*)__a))))' to 'bool'
> >
> >
> > Thanks for some info,
> > Helmut.