** Listing 3:** A better expression template

// Replaces EtMatrixAdd of Listing 2 template< typename T, size_t n, size_t m, typename LeftOp, typename RightOp> class EtMatrixAdd { public: EtMatrixAdd(const LeftOp& lhs, const RightOp& rhs) : m_lhs(lhs), m_rhs(rhs) {} T ElementAt(size_t n, size_t m) const { return m_lhs.ElementAt(n, m) + m_rhs.ElementAt(n, m); } private: const LeftOp& m_lhs; const RightOp& m_rhs; }; // Replaces operator= of Listing 2 template< typename T, size_t n, size_t m, typename LeftOp, typename RightOp> Matrix<T, n, m>& operator=( Matrix<T, n, m>& lhs, const EtMatrixAdd<T, n, m, LeftOp, RightOp>& rhs ) { for(int i=0; i<n; ++i) for(int j=0; j<m; ++j) lhs.ElementAt(i,j) = rhs.ElementAt(i,j); return lhs; } // Replaces operator+ of Listing 2 template<typename T, size_t n, size_t m> inline EtMatrixAdd<T, n, m, Matrix<T, n, m>, Matrix<T, n, m> > operator+( const Matrix<T, n, m>& lhs, const Matrix<T, n, m>& rhs ) { return EtMatrixAdd<T, n, m, Matrix<T, n, m>, Matrix<T, n, m> >( lhs, rhs ); } // Second operator+ template< typename T, size_t n, size_t m, typename LeftOp, typename RightOp > inline EtMatrixAdd< T, n, m, EtMatrixAdd<T, n, m, LeftOp, RightOp>, Matrix<T, n, m> > operator+( const EtMatrixAdd<T, n, m, LeftOp, RightOp>& lhs, const Matrix<T, n, m>& rhs ) { return EtMatrixAdd<T, n, m, EtMatrixAdd<T, n, m, LeftOp, RightOp>, Matrix<T, n, m> >(lhs, rhs); }