Efficent Sum and Assignment operator overloading in C++ - c++

Question

Hi I'm implementing a matrix class in c++
I know that there are great libraries that do that like opencv but I need to do that myself.
For example if I implement the sum I can do like this
class Mat{
public:
double* data;
int rows,cols;
Mat(int r,int c):rows(r),cols(c){
data = new double[r*c];
}
};
void Sum(Mat& A,Mat& B,Mat& C){
for (int i = 0; i < A.rows*A.cols; ++i){
C.data[i] = A.data[i]+B.data[i];
}
}
int main(){
//Allocate Matrices
Mat A(300,300);
Mat B(300,300);
Mat C(300,300);
//do the sum
sum(A,B,C);
}
I would like to get something more readable like this but without losing efficiency
C = A + B
This way C is reallocated every time and I don't want that
Thank you for your time

Answer 1

Delay the calculation.
class MatAccess {
friend class Mat;
friend class MatOpAdd;
virtual double operator[](int index) const = 0;
};
class MatOpAdd: public MatAccess {
friend class Mat;
private:
const MatAccess& left;
const MatAccess& right;
MatOpAdd(const MatAccess& left, const MatAccess& right):
left(left), right(right) {}
double operator[](int index) const {
return left[index] + right[index];
}
};
class Mat: public MatAccess{
public:
double* data;
int rows,cols;
Mat(int r,int c):rows(r),cols(c){
data = new double[r*c];
}
MatOpAdd operator +(const MatAccess& other) {
return MatOpAdd(*this, other);
}
const Mat& operator = (const MatAccess& other) {
for(int i = 0; i < rows*cols; ++i) {
data[i] = other[i];
}
return *this;
}
private:
double operator[](int index) const {
return data[index];
}
double& operator[](int index) {
return data[index];
}
};
int main(){
//Allocate Matrices
Mat A(300,300);
Mat B(300,300);
Mat C(300,300);
//do the sum
C = A + B;
}
Now the '+' calculation will be done in the "operator="
Things I would change:
MatAccess should include the dimensions (rows,cols).
Mat adding constructors and operator= or make it not copyable
Mat::operator+ and Mat::operator= check for equal rows,col
delete memory when not used anymore or
use std::vector for simpler memory managment.
Created a bigger example here: https://gist.github.com/KoKuToru/1d23af4bbf0b2bc89893