invalid operands to binary expression when overloading a operator

Question

I'm trying to write a c++ program which implements a simple Matrix with mul and add operator overloading. My matrix is stored in a simple Vector. I also overloaded the () operator to easily store the values in my Matrix.

template < typename T1, int rows, int cols>  
class Matrix{
public:
int row = rows;
int line = cols;
vector<T1> local_matrix;
Matrix(){
    local_matrix = vector<T1>(rows*cols, 0);
}
T1 &operator () (int i, int j){
    return local_matrix.at(i * cols +j);
}
const Matrix<T1, rows, cols> operator* (const Matrix<T1, rows, cols> &other)     const{
    Matrix<T1, other.row, other.line> result; // store the result here
    int sum_elements_i = 0;
    int left_counter = 0; // counter to iterate over the left Matrix
    int right_counter = 0; // and for the right one
    for(int i = 0; i < rows -1;++i){
        for(int j = 0; j < other.spalte -1; ++j){
            sum_elements_i = 0;
            for(int k = 0; k < other.reihe -1; ++k){
                sum_elements_i += local_matrix.at(left_counter) * other.local_matrix.at(right_counter);
                left_counter++; // get the next value of the left matrix
                right_counter += other.line; // and the next one of the right here we need to skip n-elements
                                            // so its the 1st/2nd/.. value of the next line
            }
            right_counter = j+1;
            result(i, j) = sum_elements_i;
        }
        left_counter = i+1;
    }
    return result;
}
};

My Problem with the * operator is that everytime i want to multiply a 4x3 with a 3x3 matrix the program crashes with the following errormsg:

invalid operands to binary expression ('Matrix<int, 4, 3>' and 'Matrix<int, 3, 3>')
E = D*A;
    ~^~

And here is the Code I'm calling my function:

Matrix<int, 3, 3> A; // Datatype, Rows, Columns 
A(0,0) = 1; A(0,1) = 2; A(0,2) = 3;
A(1,0) = 1; A(1,1) = 2; A(1,2) = 3;
A(2,0) = 1; A(2,1) = 2; A(2,2) = 3;

Matrix<int, 4, 3> D, E;
D(0,0) = 1; D(0,1) = 2; D(0,2) = 3;
D(1,0) = 4; D(1,1) = 5; D(1,2) = 6;
D(2,0) = 7; D(2,1) = 8; D(2,2) = 9;
D(3,0) = 7; D(3,1) = 8; D(3,2) = 9;

std::cout << std::endl << "D:\n" << D << std::endl;

E = D*A;

Any help would be much appreciated, thanks.

Answer 1

you have to use correct matrix size for operator * :

template <typename T, std::size_t N, std::size_t K, std::size_t M>
Matrix<T, N, M> operator* (const Matrix<T, N, K>& lhs, const Matrix<T, K, M>& rhs);

Answer 2

The way you have defined your function, your function has the signature Matrix<int, 4, 3> Matrix<int, 4, 3>::operator(const Matrix<int, 4, 3> &) const which means that you would have to try to multiply a 4 by 3 matrix with a 4 by 3 matrix and that it would somehow give you another 4 by 3 matrix. What you want instead, at minimum, is to declare your operator* as

    template <int cols2>
    Matrix<int, rows, cols2> operator*(const Matrix<int, cols, cols2> & other) const {...}

That will allow you multiply, for example, a 4 by 3 matrix with a 3 by 3 matrix and that will correctly output a 4 by 3 matrix.

Another change you can make to allow more types of matrices to be multiplied together is to make the signature

  template <class T2, int cols2>
  Matrix<typename std::common_type<T, T2>::type, rows, cols2> 
      operator*(const Matrix<T2, cols, cols2> & other) const {...}