Question

I’ve created a templated class, and it works fine:

( I’m using openCV library, so I have cv::Mat type of matrices)

template < class T, T V >
class MatFactory
{

  private:

    const T static VALUE = V;

  public:

    void create(/* some args */)
    {
      cv::Mat M;

      // filling the matrice with values V of type T

      // loop i,j
      M.at<T>(i,j) = V;

      return M;
    }
};

But later in code, I need to get an element of matrix M at some indexes (i,j). But how do I know the type T?

MatFactory<int, 1> MF;

// getting object
cv::Mat m = MF.create();

// then I need to get some value (with `.at<>` method)
int x = m.at<int>(2,3);

// But, it means I should every time explicitly declare INT type
// Can I somehow get the type, that was passed to template factory
// Than i'll write something like this:
T x = m.at<T>(2,3);

// How to get T from the deferred template?

Answer 1

Just add a type member to your MatFactory:

template <typename T, T V>
class MatFactory {
public:
    typedef T type;
   ...
};

Note that non-type template arguments are rather limited. In particular, floating point types are not allowed (thinking of it this may have been changed with C++2011).