重载运算符'+'和'='C ++

Question

I've recently started learning C++, right now I'm working on a Matrix Class. I'm trying to overload operators and it turned out to be more difficult than I thought. So I've overloaded '=' and '+', first one works perfectly fine when I just want to set one matrix equal to another, but when I do something like 'matrix = matrix1 + matrix2' it crashes with no error. I'd be very thankful if someone helps me. Here's my code:

class Matrix
{
private:
    int lines , columns;
    int *Matrix_Numbers;
public:
    Matrix();

    Matrix(int n , int m)
    {
        lines = n , columns = m;
        Matrix_Numbers = new int[lines * columns];
    }

    Matrix & operator = (Matrix &mat);

    Matrix & operator + (Matrix &mat);

    ~Matrix()
    {
        delete Matrix_Numbers;
    }
};

Matrix & Matrix::operator = (Matrix &mat)
{
    this -> lines = mat.lines;
    this -> columns = mat.columns;
    int i , j;
    for(i = 0 ; i < lines ; i++)
    {
        for(j = 0 ; j < columns ; j++)
        {
            this -> Matrix_Numbers[i * (this -> columns) + j] = mat(i , j);
        }
    }
    return *this;
}

Matrix & Matrix::operator + (Matrix &mat)
{
    Matrix result(lines , columns);
    if(mat.lines == lines && mat.columns == columns)
    {
        int i , j;
        for(i = 0 ; i < lines ; i++)
        {
            for(j = 0 ; j < columns ; j++)
            {
                result.Matrix_Numbers[i * columns + j] = Matrix_Numbers[i * 
                                            columns + j] + mat(i , j);
            }
        }
    }
    else
    {
        cout << "Error" << endl;
    }
    return result;
}

Of course it's just a part of my code, there's more, but I thought that this is the broken part. Let me know if you need more information:)

Answer 1

Your operator+ method returns a reference to the result. The only problem is that result is a local variable which means it gets destroyed upon the method returning. You should return it by value instead and let the compiler optimize stuff.

Like others pointed out in the comments, you should use const whenever possible so your operator+ can actually be called with constant parameters.