Spin problem in ur_10 class

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Hi, I have a class position.cpp where I manipulate my robot ur_10 and in this class I must to launch spinner.start()

In this class I would to subscribe to a topic, and for doing this and I wrote this class:

class ListMsg {
private:
    ros::Subscriber sub;
    ros::NodeHandle n;
public:
ListMsg(){
  ros::Subscriber sub = n.subscribe ("chatter", 500, &Subscribe_And_Publish::callback, this);
  ros::spin();
}

void callback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

};

From my position.cpp I call this class in this way:

ListMsg listMsg1;

The problem is that my class doesn't print any information. How can I resolve it?

Thank you all.