Re: [eigen] proposal to use static const integer class members instead of enum values

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Mon, 15 Dec 2014, Christoph Hertzberg wrote:

> On 15.12.2014 09:17, Daniel.Vollmer@xxxxxx wrote:
> > Well, wouldn't you then just have all these constants be derived from
> > integral_constant (in C++11-speak) and then call IsColVector::value etc.?
>
> I'm not sure if I understand your suggestion correctly, but we definitely 
> can't do this (reduced example):
>
> template<class Scalar, int Rows, int Cols>
> class Matrix {
>  Scalar[Rows*Cols];
>  integral_constant<Rows> RowsAtCompileTime;
>  integral_constant<Cols> ColsAtCompileTime;
> };
>
> Not only because the API will change, but more crucially because we'd add an

Note that integral_constant<T,x> is implicitly convertible to T.

> empty class as a member which will cause the class to get bigger (Try out the 
> above and check sizeof(Matrix<...>), sizeof(integral_constant<...>), etc. We 
> have a similar effect in Maps with fixed sized dimensions, but we don't care 
> too much at the moment, because Maps are most of the time only used as 
> temporaries.

I thought he meant:

  typedef integral_constant<int,Rows> RowsAtCompileTime;

but it still changes the API.

--
Marc Glisse