加法运算符重载不起作用

Question

I am having trouble getting my addition overloading function too work properly and was wondering if I could get some help with it. The rest of the functions and constructors in the class are default and can not be changed for this project so regardless they are correct. This means the only problems I am having are with the operator overloading functions themselves. Thanks in advance.

Matrix.h:

class Matrix {
// models a matrix of two dimentions, i. e. rows and columns of values

public:
    Matrix & operator=(const Matrix& m);

    Matrix & operator+(const Matrix& m);

Matrix.cpp:

//not included are the default and copy constructors plus read and write    
//functions, etc...
Matrix& Matrix::operator=(const Matrix& m) {
    this->rows = m.rows;
    this->cols = m.cols;
    matrix = vector< vector<double> >(rows);
    for (int r=0; r<rows; r++)
        matrix[r] = vector<double>(cols);
    for (int r=0; r<rows; r++)
        for (int c=0; c<cols; c++)
            matrix[r][c] = m.matrix[r][c];
    return *this;
}

Matrix & Matrix::operator+(const Matrix& m) {
    Matrix newMatrix;
    newMatrix = vector< vector<double> >(rows);
    if (this->rows != m.rows || this->cols != m.cols) {
        newMatrix.rows = 0;
        newMatrix.cols = 0;

        return newMatrix;
    }
    else {
        newMatrix.rows = m.rows;
        newMatrix.cols = m.cols;
        for (int r = 0; r < m.rows; r++) {
            newMatrix.matrix[r] = vector<double>(m.cols);
        }

        for (int r = 0; r < m.rows; r++) {
            for (int c = 0; c < m.cols; c++)
                newMatrix.matrix[r][c] = matrix[r][c] + m.matrix[r][c];
        }
        return newMatrix;
    }
}

Main.cpp:

//in main.cpp I am trying to do the following operation and then output the result:
e = a + b;

Answer 1

While assignment operators should return a reference to the left object (resp. *this ), all other infix operators should return by value. They are most easily written using the respective compound-assignment operator like += .
For those reason most infix non-assignment-operators are free functions.

General pattern for infix +:

template<typename T> inline const T operator(const T& a, const T& b) {
    T c = a;
    return c += b;
}

You do not want to have such infix-operators as member-functions, because that breaks symmetry.
Instead, just define the compound operator += .

Answer 2

You're returning a reference to the temporary newMatrix

Instead, your addition operator should return the result by value.

The function signature should look like this:

Matrix operator+(const Matrix& m);