Last node handle goes out of scope but callbacks still work. Why?

edit

I thought I need to keep my node handle alive by storing it in my class member, but when I make it a local variable in my constructor and let it go out of scope, my callback still gets called.

Is this proper defined behavior?

#include <ros/ros.h>
#include <std_msgs/Int32.h>

class MyNode
{

public:

    MyNode()
    {
        ros::NodeHandle nh; // will go out of scope
        subscriber = nh.subscribe("topic_name", 10, &MyNode::callback, this);
    }

private:

    void callback(std_msgs::Int32ConstPtr const & msg)
    {
        ROS_ERROR_STREAM("Callback got called");
    }

    // I thought this was mandatory
    // ros::NodeHandle nh;

    ros::Subscriber subscriber;

};

int main(int argc, char ** argv)
{
    ros::init(argc, argv, "my_node");
    MyNode node;
    ros::spin();
    return 0;
}

UPDATE:

Here it says "when the last ros::NodeHandle is destroyed, it will call ros::shutdown()". Further down it is suggested, that ros::shutdown() will make ros::spin() return. So what's up with that?