Overloading [] operator c++ - c++

Question

I am trying to overload the indexing operator for a c++ class but I am not able to do so. When I try to index my Matrix class, I get the following error:
error: cannot convert 'Matrix' to 'double*' in initialization
This error occurs on the 9th line of my main.cpp. Does it seem that the indexing does not seem to be recognized by the compiler?
Below is my code:
Matrix.h
#ifndef MATRIX_H
#define MATRIX_H
#include <iostream>
using namespace std;
class Matrix
{
public:
/** Default constructor */
Matrix(unsigned int num_cols, unsigned int num_rows);
/** Default destructor */
virtual ~Matrix();
/** Access num_cols
* \return The current value of num_cols
*/
unsigned int getCols() { return _num_cols; }
/** Access num_rows
* \return The current value of num_rows
*/
unsigned int getRows() { return _num_rows; }
double operator[](unsigned int index);
protected:
private:
unsigned int _num_cols; //!< Member variable "num_cols"
unsigned int _num_rows; //!< Member variable "num_rows"
double ** _base;
};
#endif // MATRIX_H
Matrix.cpp
#include "Matrix.h"
Matrix::Matrix(unsigned int num_cols, unsigned int num_rows){
_num_cols = num_cols;
_num_rows = num_rows;
if(_num_cols > 0) {
_base = new double*[_num_cols];
for(unsigned int i = 0; i < _num_cols; i++) {
_base[i] = arr;
cout << _base[i] << endl;
}
}
}
double* Matrix::operator[](int index) {
if (index >= _num_cols) {
cout << "Array index out of bound, exiting";
exit(0);
}
return _base[index];
}
Matrix::~Matrix()
{
//dtor
}
main.cpp
#include <iostream>
#include "Matrix.h"
using namespace std;
int main()
{
Matrix * m = new Matrix(1,2);
double * d = m[1];
delete m;
return 0;
}

Answer 1

The declaration of the overloaded operator member function doesn't match the definition.
You defined the operator as:
double* Matrix::operator[](int index) {
But declared it as:
double operator[](unsigned int index);
The declaration should be:
double *operator[](int index);
Also, the problem with this line:
double * d = m[1];
Is that m is a pointer to a Matrix and the [] operator works on the class instance, not a pointer to it, so you need to dereference m:
double * d = (*m)[1];
Or you can define m as an instance of a Matrix:
Matrix m(1,2);
double * d = m[1];