Use subscriber callback to modify class attributes

edit

Hi, I am new to ROS, and I am encountering a weird issue currently. Any help would be greatly appreciate. I use a subscriber to receive message from mavros and hope to get values from subscriber callback that used to modify private class variables. Here is my code:

class Listener{
    private:
       geometry_msgs::Vector3 position;
    public:
       void Listen_From_MAVROS_Callback(const mavros_msgs::DRONESENSOR::ConstPtr& dronesensor);
       void Print_Variables();
}
void Listener::Listen_From_MAVROS_Callback(const mavros_msgs::DRONESENSOR::ConstPtr& dronesensor)
{
      position.x = (dronesensor -> position)[0];
 }
void Listener::Print_Variables() {
     std::cout << "x position: " << position.x << std::endl;
}
int main(int argc, char **argv){
    ros::init(argc, argv, "GeoCtrlListener");
   ros::NodeHandle n;
   Listener listener;
   ros::Subscriber Sub_From_Mavros = n.subscribe("/mavros/odroid/odroid_input", 1, 
   &Listener::Listen_From_MAVROS_Callback, &listener);
  listener.Print_Variables();
   ros::spin();
  std::cout << "connection end" << std::endl;
 listener.Print_Variables();
return 0
}

For the code above when I run it, the print statement before spin() all returns 0, meaning no attibutes is modified. However, when I press ctrl c and end the connection, the console returns the correct value. I am so confused whether my object listener is correctly mofidied or not. I need to store the data from callback function and makes some manipulation and then publish back to another topic. Ayn suggestions? Thank you so much