example: pubsub

Examples for Publisher / Subscriber

The Publisher / Subscriber pattern is implemented using the classes Publisher and Subscriber.
A Publisher simply sends out messages to all Subscribers (if any) that are listening to its ip address and port.
Here is an example of a Publisher for messages of type simple_msgs::Point, it will publish 10 times to any listeners on port 5555.

int main() {
  simple_msgs::Point my_point{1.0, 2.0, 3.0};
  simple::Publisher<simple_msgs::Point> publisher{"tcp://*:5555"};   
  for (auto i = 0; i < 10; ++i) {
    publisher.publish(my_point);
  }
}

The construction of a Publisher requires to give a string as parameter. That is identical to the interface used by ZeroMQ. In this case we use:

tcp://*:5555

Which means:

• Use tcp:// as protocol.
• Send messages to Subscribers connect from * any IP address.
• Use the port 5555.

You can change this parameter accordingly to your setup.

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A Subscriber listens to messages sent by a Publisher on the given ip address and port.
Every message that arrives will be handled (asynchronously) by a given callback function. The function could do anything you need, in this example it just prints out the content of the message.

void example_callback(const simple_msgs::Point& p)
{
  std::cout << p << std::endl;
}

int main()
{
  simple::Subscriber<simple_msgs::Point> subscriber{"tcp://127.0.0.1:5555", example_callback};
}

To construct a Subscriber we used a string and the callback function. In this case we used:

tcp://127.0.0.1:5555

Which means:

• Use tcp:// as protocol.
• Connect to a Publisher on the IP: 127.0.0.1 (localhost).
• Connect to a Publisher listening on port 5555.

You can change this parameter accordingly to your setup.

The callback function:
in this small example is just a global function, but it can be anything that binds to an std::function, that is a global function, a lamba function, a class member function, etc.
For example, if you want to pass a class member function you would need to write something similar to:

class MyClass {
    MyClass(const std::string& address) subscriber_{address, std::bind(&MyClass::memberCallback, this, std::placeholders::_1)} {}
    void memberCallback(const simple_msgs::Point& received_point) { ... }
private:
    simple::Subscriber<simple_msgs::Point> subscriber_;
};    

Full Example

A full example is available for a Publisher and a Subscriber.
The example will exchange PoseStamped messages over the localhost.