Get an array of pointers to arrays
M::mtx_item_type** operator M::mtx_item_type**( );
This overloaded operator returns an array of pointers to the rows or columns of the matrix.
Because the pointers are not const, the mtx_ptrreader<M> makes a copy of the data contained in the attached matrix, and returns pointers to the copied data.
Thus the data in the original matrix is never affected by write operations to the passed arrays.