The parameters to add_matrices (in both parts) have type const matrix_type&. Why would this type be a better choice than the plain matrix_type type?

The print_matrix function in both parts uses for-each style loops (e.g. for (auto x: M)) instead of the more traditional index-based loops (e.g. for( i = 0; i < n; i++ )). Can the add_matrices function be written with for-each loops? Why or why not?