Reducing template parameter with using or typedef

Question

I'm trying to increase my code readabiliy by introducing the "using" keyword.

namespace EigenRM
{
    template<typename T>
    using MatrixX<T> = Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>;
}

This code is not working. I've seen examples using "using" to remove all template parameters but none for retaining one. Is this even possible?

Answer 1

Try removing the <T> after MatrixX

template<typename T>
using MatrixX<T> = Eigen::Matrix<T, ...
// wrong ....^^^

If you precede the using definition of a name foo with a template declaration, it's implicit that you're defining the template argument over foo, so simply

namespace EigenRM
{
    template<typename T>
    using MatrixX = Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>;
}

-- EDIT --

The OP says

This is exactly what i am not trying to do. I am trying to write EigenRM::MatrixX<double> //instead of Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> within an untemplated function

This is exactly what you get if you remove the "<T>": EigenRM::MatrixX<double> become an alias for Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>.

I'm without an Eigen installation but the following example should explain what I mean

#include <type_traits>

template <typename, typename, typename>
struct foo;

template <typename T>
using bar = foo<T, float, int>;

int main ()
 {
   static_assert(std::is_same< bar<double>,
                               foo<double, float, int> >{}, "!");
 }