C++ overloading operators for matrix structure - c++

Question

I am trying to make matrix structure with overloading operators '+' and '='.
Matrix structure:
struct Matrix{
int rows;
int cols;
std::vector<int> data;
Matrix(int rows, int cols, int val); //creates matrix row*col and fills every position with val
Matrix(const Matrix &m); //copy constructor
Matrix(int rowcol = 0); //creates matrix rowcol*rowcol filled with zeros
Matrix & operator=(const Matrix &m);
void printMatrix(); //prints the matrix
...
};
Operators:
Matrix & Matrix::operator=(const Matrix &m) //assings matrix m to the new matrix
{
}
Matrix operator+(const Matrix &a, const Matrix &b) //sums two matrices
{
//I know how to make the algorithm for summing two matrices, however i don´t know how
//to return the result into existing matrix
}
And main function:
int main(){
Matrix A(3,3,1);
Matrix B(3,3,2);
Matrix C(3,3,0);
C = A + B;
C.printMatrix();
...
return 0;
}
I don´t know how to make it work. I tried something like this for the '=' operator:
Matrix & Matrix::operator=(const Matrix &m)
{
static matrix d(m); //using copy constructor
return d;
}
but it didn´t work. It created new matrix with dimensions 0x0. Any advice on how to implement it?

Answer 1

A working way to implement that operator= would be as follows:
Matrix& Matrix::operator=(const Matrix &m)
{
this->rows = m.rows;
this->cols = m.cols;
this->data = m.data;
return *this;
}
I won't approach the algorithm needed for operator+, but the surrounding machinery is also surprisingly simple:
Matrix operator+(const Matrix &a, const Matrix &b) //sums two matrices
{
Matrix result(<num of rows>, <num of cols>, 0);
// do things with result here, using your algorithm, a, and b
// e.g. result.data[?] = a.data[?] ? b.data[?]
return result;
}
To make this easier, since your data is one-dimensional (good! this is efficient!), you may want to implement an accessor that takes x and y dimensions and returns an index into data. For example:
int& Matrix::val(const int x, const int y)
{
assert(x < cols);
assert(y < rows);
return data[x + y*cols];
}
The result of this may be both read and written, like this:
result.val(x1, y1) = a.val(x2, y2) + b.val(x3, y3);
Where those co-ordinates and the + are just things I made up, and should come from your algorithm.
If you also want an operator+=, to affect an existing matrix, then:
Matrix& Matrix::operator+(const Matrix &b) //sums two matrices
{
// do things with your object here, using your algorithm and b
return *this;
}
Where I return *this for the member operators, that's not strictly necessary, though it is conventional, and it allows for easy chaining of operations (if you're into that kind of thing). You can find more information on operator overloading in your book.