Re: [eigen] Floating point as index type

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi!

If we wanted to ensure not to break existing code here, we'd need to 
explicitly allow everything which can be converted to Eigen::Index (aka 
std::ptrdiff_t). But on the one hand, that would be complicated (due to 
the new Indexing feature), and on the other hand, I agree that this 
might rather hide existing bugs. (Like accessing Vector[(double(N)/n)*n] 
may or may not round down)

As a workaround you could write yourself a DenseBase plugin which adds 
`operator()(double, double)`, etc:
https://eigen.tuxfamily.org/dox-devel/TopicCustomizing_Plugins.html

But as also suggested by Rasmus, you should really just explicitly cast 
your floats/doubles to Eigen::Index (perhaps after rounding instead of 
truncating -- depending on your use-case). This will not add any 
overhead, as this was happening implicitly before -- only some syntactic 
noise which actually is good, since you are misusing types here.


Cheers,
Christoph




On 25/11/2019 23.16, Gael Guennebaud wrote:
> Hi,
>
> yes we are aware of that. We could explicitly accept float and double as
> "valid_index_type", but I think this would just keep silent some latent
> bugs. I've just added note there:
> http://eigen.tuxfamily.org/index.php?title=3.4
>
> Cheers,
> gael
>
> On Mon, Nov 25, 2019 at 7:57 PM Janos Meny <janos.meny@xxxxxxxxxxxxxx>
> wrote:
>
> > Hey everyone,
> >
> > I noticed that in the development branch the following code will no longer
> > compile: https://godbolt.org/z/pHPX2j
> >
> > Aside from the fact that I think this is a bug, this would break a lot of
> > code I think.
> >
> > Best regards
> >
> > Janos


--
 Dr.-Ing. Christoph Hertzberg

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