c++ overload operator() for assigning value in a dynamic 2D array

I am trying to overload () operator to assign value into a dynamically allocated 2D array, here is my code --

class test {
    private:
        int** data ; int row, col ;

    public:
        test(int row = 2, int col = 2)  {
            this->row = row ; this->col = col ;
            this->data = new int*[this->row] ;
            for(int i = 0 ; i < this->row ; i++)
                this->data[i] = new int[this->col] ;
        }

        ~test() {
            for(int i = 0 ; i < this->row ; i++)
                delete [] this->data[i] ;
            delete [] this->data ;
        }

        const int operator() (int row, int col) { // read operation
            return this->data[row][col] ;
        }

        int& operator() (int row, int col) { // write operation
            return this->data[row][col] ;
        }

        // for printing
        friend ostream& operator<< (ostream &os, const test &t);
};

In the operator() write operation, I am trying to return the value by reference so that I can assign value like this --

test t(4,4) ;
t(2,2) = 5 ;

But it does not compile, says that I can't do such kind of overloading, any idea ?