subscribe and publish in the same class

class Spinner {
  ros::NodeHandle n_;
  ros::Publisher spin_pub;
  ros::Subscriber sub;
public:
  Spinner(ros::NodeHandle n) :
    n_(n)
  {
    spin_pub = n.advertise<geometry_msgs::Twist> ("/turtlebot_node/cmd_vel", 1000);
    sub = n.subscribe("/robot_pose_ekf/odom_combined", 1000,  spin_cb);  //Does not work!
  }

  void spin_cb(const geometry_msgs::PoseWithCovarianceStamped msg)
  {
    //do stuff
  }
};

int main(int argc, char** argv)
{
  ros::init(argc, argv, "spin");
  ros::NodeHandle n;
  Spinner spin_object = Spinner(n);
  ros::spin();
  return 0;
}