C++矩阵运算题，求大神解答

#include <iostream>

template<typename T, size_t ROW, size_t COL>
class Matrix;

template<typename T, size_t ROW, size_t COL>
std::ostream& operator<<( std::ostream& os, const Matrix<T,ROW,COL>& m );

template<typename T, size_t ROW, size_t COL>
class Matrix
{
public:
    Matrix()
    {
        for( size_t r=0; r!=ROW; ++r )
            for( size_t c=0; c!=COL; ++c )
                value_[r][c] = T();
    }
    Matrix( const std::initializer_list<T>& list )
    {
        auto itor = std::begin(list);
      

        for( size_t r=0; r!=ROW; ++r )
            for( size_t c=0; c!=COL; ++c )
                if( itor != std::end(list) )
                    value_[r][c] = *itor++;
                else
                    value_[r][c] = T();
    }

    Matrix operator+( const Matrix& m ) const
    {
        Matrix ret;
        for( size_t r=0; r!=ROW; ++r )
            for( size_t c=0; c!=COL; ++c )
                ret.value_[r][c] = value_[r][c]+m.value_[r][c];
        return ret;
    }
private:
    T value_[ROW][COL];

    friend std::ostream& operator<< <T,ROW,COL> ( std::ostream& os, const Matrix<T,ROW,COL>& m );
};

template<typename T, size_t ROW, size_t COL>
std::ostream& operator<<( std::ostream& os, const Matrix<T,ROW,COL>& m )
{
    for( size_t r=0; r!=ROW; ++r )
    {
        for( size_t c=0; c!=COL; ++c )
            os << m.value_[r][c] << '\t';
        os << '\n';
    }
    return os;
}

using namespace std;

int main( void )
{
    Matrix<int,3,3> m1 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    Matrix<int,3,3> m2 = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    cout << m1 << endl;
    cout << m2 << endl;
    cout << m1+m2 << endl;

    return 0;
}

1       2       3
4       5       6
7       8       9

1       2       3
4       5       6
7       8       9

2       4       6
8       10      12
14      16      18