[mlpack] Aliasing to object with templated type and size

[Ryan Curtin]

On Mon, Sep 02, 2019 at 01:45:54PM -0700, Joe Dinius wrote:
> Ryan's really helped me out so far, so I'm hoping he can do the same for
> this simple issue I'm having.
> 
> I'd like to be able to allocate my own fixed size vector-type using
> aliasing.  The following code works well for this:
> 
> template<size_T N>
> using Vector = typename arma::colvec::fixed<N>;
> 
> Additionally, it would be really handy If I could add the *type* to the
> template definition as well; something like this:
> 
> template<typename T, size_T N>
> using Vector = typename arma::Col<T>::fixed<N>;
> 
> However, this *does not* work; it fails to compile.  This seems more an
> issue with how I am abusing the 11-standard and not an issue with
> armadillo, but I am hoping that someone on this list can shed some light on
> what I am doing wrong here:  how can I achieve the desired design pattern
> "Vector<double, N>" is an armadillo vector of doubles of fixed size N.

Hey Joe,

To solve this issue you need to use the ::template operator.  (Or, at
least, I was able to solve it this way.)

template<typename T, size_t N>
using Vector = typename arma::Col<T>::template fixed<N>;

More information on this headachey part of C++ here:

https://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords

Hope this helps!

Ryan

-- 
Ryan Curtin    | "No... not without incident."
ryan at ratml.org |   - John Preston