C++重载运算符

class Point3d {
private:
    float _x;
    float _y;
    float _z;
public:
    Point3d(float x=0.0,float y=0.0,float z=0.0):_x(x),_y(y),_z(z){}

    //重载<<
    friend ostream& operator<<(ostream &os, const Point3d &pt) {
        return os << pt._x << " , " << pt._y << " , " << pt._z << endl;
    }

    //重载>>
    friend istream& operator>>(istream &is,Point3d &pt) {
        is >> pt._x >> pt._y >> pt._z;
        return is;
    }

    //重载[]
    float operator[](int index) {
        if (index < 0 || index>2) {
            cout << "error" << endl;
        }
        else {
            if (index == 0) {return _x;}
            if (index == 1) {return _y;}
            if (index == 2) { return _z; }
        }
    }
};

int main() {
    Point3d p;
    cout << p << endl;;
    cin >> p;
    cout << p << endl;;
    cout << p[0] << endl;
}