如何在模板中编写朋友(操作员）方法

因此，我正在尝试在模板类中超载操作员 。代码编译和运行，但在使用操作员 时崩溃。尝试了很多事情，我认为这是语法问题吗？任何意见，将不胜感激！

运算符=超载并有效。

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;
}

如果您需要代码的其他部分，请告诉我！预先感谢！