...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
static inline scalar_type & write_element_idx( int r, int c, Matrix & m );
This function is expected to be static member of user-defined specializations of the mat_traits template. It returns mutable reference to the element at row r and column c of m.