How to write a friend (operator) method in a template? - c++

Question

So I'm trying to overload the operator + in a template class.
the code compiles and runs but crashes at the use of the operator +.
tried so many things, I think it's a syntax problem? any advice would be appreciated!
The operator = is overloaded and works.
Matrix.h
template <int row, int col, class T = int>
class Matrix
{
int rows;
int cols;
T** mat;
public:
Matrix(int defVal = 0) {
rows = row;
cols = col;
memory();
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
mat[i][j] = defVal;
}
~Matrix() {
del();
}
Matrix(const Matrix& other) {
*this = other;
}
const Matrix& operator=(const Matrix& other) {
if (&other != this)
{
rows = other.rows;
cols = other.cols;
del();
memory();
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
mat[i][j] = other.mat[i][j];
}
return *this;
}
friend ostream& operator<<(ostream& os, const Matrix& m) {
for (int i = 0; i < m.cols; i++)
{
for (int j = 0; j < m.rows; j++)
os << m.mat[i][j] << " ";
os << endl;
}
return os;
}
friend Matrix operator+(const Matrix& other, T num) {
Matrix temp = other;
for (int i = 0; i < temp.rows; i++)
for (int j = 0; j < temp.cols; j++)
temp.mat[i][j] += num;
return temp;
}
void memory(){
mat = new T * [rows];
for (int i = 0; i < rows; i++)
mat[i] = new T[cols];
}
void del(){
for (int i = 0; i < rows; i++)
delete[] mat[i];
delete[] mat;
}
};
main.cpp
int main() {
Matrix<4, 4> mat;
std::cout << mat << std::endl;
Matrix<4, 4> identity(1);
std::cout << identity + 3 << std::endl; //crashes here
return 0;
}
if you need other parts of the code let me know! Thanks in advance!

Answer 1

Your copy constructor and assignment looks suspicious, you del after changing rows and cols, without initialising anything.
I expect it should be
Matrix(const Matrix& other) : rows(0), cols(0), mat(nullptr) {
*this = other;
}
const Matrix& operator=(const Matrix& other) {
if (&other != this)
{
del(); // clean up first
rows = other.rows;
cols = other.cols;
memory();
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
mat[i][j] = other.mat[i][j];
}
return *this;
}
As an aside, I wouldn't use dynamic allocation here at all, and instead
template <typename T, size_t rows, size_t cols>
class Matrix
{
std::array<std::array<T, cols>, rows> mat;
public:
Matrix(T defVal = {}) {
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
mat[i][j] = defVal;
}
friend std::ostream& operator<<(std::ostream& os, const Matrix& m) {
for (int i = 0; i < cols; i++)
{
for (int j = 0; j < rows; j++)
os << m.mat[i][j] << " ";
os << std::endl;
}
return os;
}
friend Matrix operator+(Matrix other, T num) {
for (int i = 0; i < rows; i++)
for (int j = 0; j < cols; j++)
other.mat[i][j] += num;
return other;
}
// No need for any special members
};