Question

I have a little bit of a problem… I understand what a EXC_BAD_ACCESS error is and I generally know how to fix it but this one has got me completely stuffed. I have this all within a class, here is one method:

double Matrix::get_element(int r, int c) const {
    //Retrieve the element at row r and column c
    //Should not modify the value stored in Matrix but return a double copy of the value

    double currentValue = matrix[r][c];
    return currentValue;
}

Now, I have another piece of my code that calls this method:

std::string Matrix::to_string() const {
    std::string result;
    double current;
    Matrix working = *this;
    std::ostringstream oss;

    oss << "[";
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            current = 0.0;
            current = working.get_element(i, j);
            oss << " " << current << " ";
        }
        oss << "; ";
    }
    oss << "]";
    result = oss.str();
    return result;
}

I know that the working object has 3 rows and 3 cols at the point where working.get_element(i, j); is called. The variable list shows me just before the get_element() method, that both rows and cols are set to 3. In the method, I’m able to get the value at get_element(0, 0) but not get_element(0, 1).

I can’t see why this is the case… Anyone know why or require more of my code to understand why these methods are being called?

EDIT:
Here is the header file:

class Matrix {
private:
    //Any variables required
    int rows;
    int cols;
    double **matrix;

public:
    Matrix();   //Working M
    ~Matrix();  //Working M
    Matrix(int r, int c);   //Working M

    int getRows();
    int getCols();

    void set_element(int r, int c, double val); //Working M
    double get_element(int r, int c) const; //Working M

    void clear(); //Working M        
    bool is_empty(); //Working M         
    bool is_identity(); //Working M

    const Matrix transpose(); //Working M
    int minorMat(double **dest, const int row, const int col, int order); //Working M
    double get_determinent(); //Working M
    double higherDeterminents(int order); //Working M

    const Matrix operator+(const Matrix &rhs); //Working M       
    const Matrix operator-(const Matrix &rhs); //Working M    
    const Matrix operator*(const Matrix &rhs); 
    bool operator==(const Matrix &rhs); //NOT assessed
    const Matrix operator*(const double &rhs);        
    const Matrix operator/(const double &rhs);
    Matrix & operator=(const Matrix &rhs);

    std::string to_string() const;
};

Do ignore the comments sorry. And this is the constructors/destructors:

Matrix::Matrix() {
    //Basic Constructor
    rows = 1;
    cols = 1;
    matrix = new double*[rows];
    for (int i = 0; i < rows; ++i) {
        matrix[i] = new double[cols];
    }
}

Matrix::~Matrix() {
    //Basic Deconstructor
    for (int i = 0; i < rows; ++i) {
        delete[] matrix[i];
    }
    delete[] matrix;
    rows = NULL;
    cols = NULL;
    matrix = NULL;
}

Matrix::Matrix(int r, int c) {
    //Empty matrix (all 0's) with r rows and c columns, if they are -ve, set to 1
    rows = r;
    cols = c;

    if (cols < 0)
        cols = 1;
    if (rows < 0)
        rows = 1;

    matrix = NULL;
    matrix = new double*[rows];
    for (int i = 0; i < rows; i++) {
        matrix[i] = new double[cols];
    }
}

EDIT2:

Matrix & Matrix::operator=(const Matrix &rhs) {
    //rhs is matrix to be copied
    //rhs compied into Matrix called on
    double toCopy;
    for (int i = 0; i < rhs.rows; i++) {
        for (int j = 0; j < rhs.cols; j++) {
            toCopy = rhs.get_element(i, j);
            this->set_element(i, j, toCopy);
        }
    }
    return *this;
}

Answer 1

It is impossible for us to say when you do not state how you declare and initialize the matrix element. Using something like that in your CTOR should be fine:

class Matrix {
   float matrix[3][3];
   ...
}

Don’t forget to initialize it in your CTOR to something that makes sense.

Btw: why do you do this: Matrix working = *this; ?? You could simply this->get_element(i, j); instead, which would not invoke copying of your whole object. [1]

EDIT: Update since you updated your answer. You should be careful with your copy CTORs and operator=() statements. It is easily possible to make double deletes or something ugly like that.

EDIT2: I think the problem is this line:

Matrix working = *this;

You are creating a new copy working of your this object. But working gets initialized with only 1 column and 1 row (as defined in your standard CTOR). I’m not sure if you are checking the bounds when calling set_element or get_element so I guess you are writing over the bounds of your arrays.

I think the best idea is to just remove the line Matrix working = *this; and to adhere to my tip in above:
this->get_element(i, j); in std::string Matrix::to_string() const.