Re: [eigen] Re: [Bitbucket] Pull request #80: Allow LevenbergMarquardt to work with non-standard types. (eigen/eigen)

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 29.08.2014 17:55, Thomas Capricelli wrote:
> Il 28/08/2014 22:31, Jitse Niesen ha scritto:
> > Sorry, I should probably have waited for your feedback instead of
> > jumping the gun and merging it.
>
> No, it's ok, you did well.
>
>
> > static Scalar sqrt_(const Scalar& x) { using std::sqrt; return sqrt(x); }
> >
> > If Scalar is a custom type like mpreal which is defined in its own
> > namespace, then this function also looks for sqrt in that namespace (I
> > think it's called argument-dependent lookup). For standard types there
> > is no difference.
>
> You mean that
>
>      { using std::sqrt; return sqrt(x); }
>
> will use std::sqrt() only when sqrt(someothertype) is not previously
> defined ?

If Scalar is defined in namespace X and X::sqrt exists, it will use 
X::sqrt, otherwise, or if Scalar is not in any namespace, it will (try 
to) fall back to std::sqrt (I never, really understood every detail, 
which rule overrules which; especially, what happens if a conflicting 
definition exists in global scope.)
wikipedia has an article about ADL (maybe the links show more details):
http://en.wikipedia.org/wiki/Argument-dependent_name_lookup

Regarding the pull-request: Wouldn't it be better to define sqrt once in 
the Eigen::numext namespace, and always use that? I don't think we 
should define a sqrt_ function in every module that needs it.

Christoph

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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
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