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Note: This tutorial assumes that you have completed the previous tutorials: Writing a Simple Publisher and Subscriber, Writing a Simple Service.
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Using Class Methods as Callbacks

Description: Most of the tutorials use functions in their examples, rather than class methods. This is because using functions is simpler, not because class methods are unsupported. This tutorial will show you how to use class methods for subscription and service callbacks.

Tutorial Level: BEGINNER

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Subscriptions

Suppose you have a simple class, Listener:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/listener_class/listener_class.cpp

  35 class Listener
  36 {
  37 public:
  38   void callback(const std_msgs::String::ConstPtr& msg);
  39 };

Where the NodeHandle::subscribe() call using a function would have looked like this:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/listener/listener.cpp

  75   ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

Using a class method looks like this:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/listener_class/listener_class.cpp

  51   Listener listener;
  52   ros::Subscriber sub = n.subscribe("chatter", 1000, &Listener::callback, &listener);

If the subscriber is inside the class Listener, you can replace the last argument with the keyword this, which means that the subscriber will refer to the class it is part of.

Service Servers

Suppose you have a simple class, AddTwo:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/add_two_ints_server_class/add_two_ints_server_class.cpp

  31 class AddTwo
  32 {
  33 public:
  34   bool add(roscpp_tutorials::TwoInts::Request& req,
  35            roscpp_tutorials::TwoInts::Response& res);
  36 };

Where the old NodeHandle::advertiseService() call would look like:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/add_two_ints_server/add_two_ints_server.cpp

  45   ros::ServiceServer service = n.advertiseService("add_two_ints", add);

The class version looks like this:

https://raw.github.com/ros/ros_tutorials/groovy-devel/roscpp_tutorials/add_two_ints_server_class/add_two_ints_server_class.cpp

  52   AddTwo a;
  53   ros::ServiceServer ss = n.advertiseService("add_two_ints", &AddTwo::add, &a);

For the full set of subscribe(), advertiseService(), etc. overloads, see the NodeHandle class documentation


2019-06-15 13:06