Why "subs_.push_back" in listener_with_userdata.cpp

edit

In the tutorial of listener, there is an example for listener_with_userdata, why we use push_back to initialise the three different callback functions.

class Listener
{
public:
  ros::NodeHandle node_handle_;
  ros::V_Subscriber subs_;

  Listener(const ros::NodeHandle& node_handle)
  : node_handle_(node_handle)
  {
  }

  void init()
  {
    subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 1")));
    subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 2")));
    subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 3")));
  }

  void chatterCallback(const std_msgs::String::ConstPtr& msg, std::string user_string)
  {
    ROS_INFO("I heard: [%s] with user string [%s]", msg->data.c_str(), user_string.c_str());
  }
};

int main(int argc, char **argv)
{
  ros::init(argc, argv, "listener_with_userdata");
  ros::NodeHandle n;

  Listener l(n);
  l.init();

  ros::spin();

  return 0;
}

For the init() part.

void init()
 {
        subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 1")));
        subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 2")));
        subs_.push_back(node_handle_.subscribe<std_msgs::String>("chatter", 1000, boost::bind(&Listener::chatterCallback, this, _1, "User 3")));
 }

I checked the code document, it seems that the line match this version of subscriber http://docs.ros.org/indigo/api/roscpp...

It seems that the push_back code matches the bellowing version.

template<class M>
Subscriber subscribe(const std::string& topic, 
  uint32_t queue_size, 
  const boost::function<void (const boost::shared_ptr<M const>&)>& callback, 
  const VoidConstPtr& tracked_object = VoidConstPtr(), 
  const TransportHints& transport_hints = TransportHints())
{
  SubscribeOptions ops;
  ops.template init<M>(topic, queue_size, callback);
  ops.tracked_object = tracked_object;
  ops.transport_hints = transport_hints;
  return subscribe(ops);
}

I know the basic demos of publish and subscribe mechanism of ROS, but I am not so familiar with the advanced C++ templates.

Q1. <std_msgs::String>is the return type of node_handle_.subscribe is that right? Is this type match the first argument of the chatterCallback function?

Q2. Why we use push_back() to initialize these three callback functions? Are they running as separated thread after push into the V_Subscriber?

Q3. I also did not catch the meaning of "On success, a Subscriber that, when all copies of it go out of scope, will unsubscribe from this topic.". What is the scope and why it will go out of scope during the callback?

Does any could explain the logic here?