Re: [eigen] Scalar vs. int / double in SelfAdjointEigenSolver.h

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2016-08-11 08:40, Pavel Holoborodko wrote:
> *...which may not the case for user defined type.*
>
>
>  From my experience of creating custom scalar type (mpreal) -
> I can say that keeping code without explicit conversions to Scalar is very
> beneficial.

I generally agree on that -- and this is also true for AutoDiff-types. 
However we traditionally did convert all POD scalars to "Scalar" before 
using it, in order to be compatible with custom scalars which do not 
support operators with mixed types.
I guess an optimal solution could be add a templated convert method, 
into NumTraits<Scalar> like:

template<class Scalar>
struct NumTraits {
  // other NumTraits stuff ...
  template<class X>
  static Scalar convert(const X& x) { return Scalar(x); }
};

which, if Scalar supports operations with PODs, can be specialized to:
  template<class X>
  static const X& convert(const X& x) { return x; }


Unfortunately, this would require massive refactoring. Maybe we just add 
the Scalar() conversion for now, and consider changing this for 3.3.x or 
3.4?


Christoph








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