C++ operator+ and operator+= overloading

Question

I'm implementing my own matrix class in c++ to help me develop my understanding of the language. I read somewhere that if you've got a working += operator, to use it in your + operator. So that's what I've got:

template <class T>
const Matrix<T>& Matrix<T>::operator+(const Matrix<T> &R){

    Matrix<T> copy(*this);
    return copy += R;
}

And here is the += operator overload:

template <class T>
const Matrix<T>& Matrix<T>::operator+=(const Matrix<T> & second_matrix){
    //Learn how to throw errors....
    if (rows != second_matrix.getNumRows() || cols != second_matrix.getNumCols()){throw "Dimension mismatch.";}
    int i,j;
    for (i = 0; i < rows; i++){
        for (j = 0; j < cols; j++){
            data[i][j] += second_matrix.get(i,j);
        }
    }
    return *this;
}

I can use the += just fine (eg, a += b; returns no errors). But calling the + operator (eg, a = b + c;) returns :

test.cpp.out(77055) malloc: *** error for object 0x300000004: pointer being freed was not allocated

Just for completeness, here's my destructor:

template <class T>
Matrix<T>::~Matrix(){
    for (int i = 1; i < rows; i++){
        delete[] data[i]; }
    delete[] data;
}

I've been using C++ for a couple years on and off, and still have trouble sometimes keeping track of pointers. I hope that's normal... Any help would be great. Thanks!

EDIT: here's my copy constructor. It was set to free the data arrays but i removed that. now I get segmentation faults.

template <class T>
Matrix<T>::Matrix(const Matrix<T>& second_matrix){

    rows = second_matrix.getNumRows();
    cols = second_matrix.getNumCols();
    data = new T*[rows];

    int i,j;
    for (i = 0; i < rows; i++){
        data[i] = new T[cols];
    }
    for (i = 0; i < rows; i++){
        for (j = 0; j < cols; j++){
            data[i][j] = second_matrix.get(i,j);
        }
    }

}

Answer 1

This is how I have implemented such operators for a Matrix class, this is based on a Vector Class. Once you define some operators all other should be defined in terms of the simplest operators:

Matrix::Matrix(const Matrix& rMatrix) :
    _iRows(rMatrix._iRows), _iColumns(rMatrix._iColumns), _pVector(0)
{
    _pVector = new Vector[_iRows];
    for (int i = 0; i < _iRows; i++) { _pVector[i] = rMatrix._pVector[i]; }
}

Matrix& Matrix::operator=(const Matrix& rMatrix)
{
    if (this != &rMatrix)
    {
        if (0 != _pVector) { delete[] _pVector; pVector = 0; }
        _iRows = rMatrix._iRows;
        _iColumns = rMatrix._iColumns;
        _pVector = new Vector[_iRows];
        for (int i = 0; i < _iRows; i++) { _pVector[i] = rMatrix._pVector[i]; }
    }
    return *this;
}
Matrix& Matrix::operator+=(const Matrix& rMatrix)
{
    *this = *this + rMatrix;
    return *this;
}

Matrix Matrix::operator+(const Matrix& rMatrix) const
{
    Matrix matrix(_iRows, _iColumns);
    ValidateSizes(rMatrix);
    for (int i = 0; i < _iRows; i++) { matrix._pVector[i] = _pVector[i] + rMatrix._pVector[i]; }
    return matrix;
}

Matrix operator+(const Matrix& rMatrix, double dNum)
{
    Matrix matrix(rMatrix._iRows, rMatrix._iColumns);
    matrix.ValidateSizes(rMatrix);
    for (int i = 0; i < matrix._iRows; i++) { matrix._pVector[i] = dNum + rMatrix._pVector[i]; }
    return matrix;
}

Matrix operator+(double dNum, const Matrix& rMatrix)
{
    return operator+(rMatrix, dNum);
}

bool Matrix::ValidateSizes(const Matrix& rMatrix) const
{
    if (_iRows != rMatrix._iRows) { /* THROW EXCEPTION */ }
    if (_iColumns != rMatrix._iColumns) { /* THROW EXCEPTION */ }
    return true;
}