订阅节点，主要经过subscribe方式。html
最多见的使用方法api
void callback(const std_msgs::Empty::ConstPtr& message) { } ros::Subscriber sub = handle.subscribe("my_topic", 1, callback);
其中subscribe有不少重定义。例如：函数
Subscriber ros::NodeHandle::subscribe ( const std::string & topic, uint32_t queue_size, void(*)(M) fp, const TransportHints & transport_hints = TransportHints() )
Parameters:ui
| M | [template] M here is the callback parameter type (e.g. const boost::shared_ptr<M const>& or const M&), not the message type, and should almost always be deduced |
| topic | Topic to subscribe to |
| queue_size | Number of incoming messages to queue up for processing (messages in excess of this queue capacity will be discarded). |
| fp | Function pointer to call when a message has arrived |
| transport_hints | a TransportHints structure which defines various transport-related options |
其中的参数： this
topic 为订阅的节点名，字符串类型。spa
queue_size 为待处理信息队列大小。ssr
fp 当消息传入时，能够调用的函数指针，即回调函数。指针
而其中M是回调函数的不一样类型，例如const boost::shared_ptr<M const>& or const M&。这样的话，咱们还能够使用boost::bind()调用回调函数。code
可是，咱们还会遇到subscribe()的第四个参数，例如：htm
Subscriber ros::NodeHandle::subscribe ( const std::string & topic, uint32_t queue_size, void(T::*)(M) fp, T * obj, const TransportHints & transport_hints = TransportHints() )
Parameters:
| M | [template] M here is the message type |
| topic | Topic to subscribe to |
| queue_size | Number of incoming messages to queue up for processing (messages in excess of this queue capacity will be discarded). |
| fp | Member function pointer to call when a message has arrived |
| obj | Object to call fp on |
| transport_hints | a TransportHints structure which defines various transport-related options |
当咱们要使用一个Class里面的返回函数以后，咱们须要调用第四个参数。例如咱们有一个class：
class Listener { public: void callback(const std_msgs::String::ConstPtr& msg); };
若是想要调用这个class里的返回函数，能够使用第四个参数，以下：
Listener listener; ros::Subscriber sub = n.subscribe("chatter", 1000, &Listener::callback, &listener);
第四个参数定义的class，这样咱们调用的就是Listener的callback函数。若是方法在class里面，能够使用this。以下：
class Listener { public: void callback(const std_msgs::String::ConstPtr& msg){}
ros::Subscriber sub = n.subscribe("chatter", 1000, &Listener::callback, this);
void TestObject(ros::NodeHandle n)
{
}
};
这样能够调用Class里的回调函数啦。
参考资料
ROS::NodeHandle ：
Using Class Methods as Callbacks：
http://wiki.ros.org/roscpp_tutorials/Tutorials/UsingClassMethodsAsCallbacks\
Using Class Methods as Callbacks issu：
https://answers.ros.org/question/207254/setting-class-method-callback-from-within-object-c/
第四个参数：