ROS C++ - Ⅱ
================

ROS publisher and subscriber nodes using C++
^^^^^^^^^^^^^^^^

Step 1: Set Up Your ROS Workspace
- Before you create nodes, make sure you have a ROS workspace set up. If you don't have one, you can create it using the following commands:

    .. code-block:: bash

        mkdir -p ~/catkin_ws/src
        cd ~/catkin_ws/
        catkin_make

Step 2: Create a ROS Package

- Inside the src directory of your ROS workspace, create a new ROS package. Replace my_cpp_package with your desired package name:

    .. code-block:: bash

        cd ~/catkin_ws/src
        catkin_create_pkg my_cpp_package roscpp std_msgs

Step 3: Create the Publisher Node

- Navigate to your package directory:
    .. code-block:: bash

        cd ~/catkin_ws/src/my_cpp_package

- Create a source file for the publisher node. For example, publisher_node.cpp:
    .. code-block:: cpp

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

        int main(int argc, char** argv) {
            // Initialize ROS
            ros::init(argc, argv, "publisher_node");

            // Create a ROS node handle
            ros::NodeHandle nh;

            // Create a publisher for the "my_topic" topic
            ros::Publisher pub = nh.advertise<std_msgs::String>("my_topic", 10);

            // Set the publishing rate
            ros::Rate rate(1);

            // Main loop
            while (ros::ok()) {
                // Create a message
                std_msgs::String msg;
                msg.data = "Hello, ROS Publisher!";
                
                // Publish the message
                pub.publish(msg);

                // Sleep to maintain the publishing rate
                rate.sleep();
            }

            return 0;
        }

Step 4: Create the Subscriber Node

- Create another source file for the subscriber node. For example, subscriber_node.cpp:
    .. code-block:: cpp

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

        // Callback function to process incoming messages
        void messageCallback(const std_msgs::String::ConstPtr& msg) {
            ROS_INFO("Received: %s", msg->data.c_str());
        }

        int main(int argc, char** argv) {
            // Initialize ROS
            ros::init(argc, argv, "subscriber_node");

            // Create a ROS node handle
            ros::NodeHandle nh;

            // Create a subscriber for the "my_topic" topic with the callback function
            ros::Subscriber sub = nh.subscribe("my_topic", 10, messageCallback);

            // Spin to receive messages
            ros::spin();

            return 0;
        }

Step 5: Update CMakeLists.txt

- Open the CMakeLists.txt file in your package directory and add the following lines to configure the build for both the publisher and subscriber nodes:

    .. code-block:: cmake

        add_executable(publisher_node src/publisher_node.cpp)
        target_link_libraries(publisher_node ${catkin_LIBRARIES})

        add_executable(subscriber_node src/subscriber_node.cpp)
        target_link_libraries(subscriber_node ${catkin_LIBRARIES})

Step 6: Build and Run

- Build your package using catkin_make:

    .. code-block:: bash

        cd ~/catkin_ws
        catkin_make

- Source the setup script:

    .. code-block:: bash

        source devel/setup.bash

- Now you can run the publisher and subscriber nodes in separate terminal windows:

    - Terminal 1 (for the publisher node):

        .. code-block:: 
            
            rosrun my_cpp_package publisher_node

    - Terminal 2 (for the subscriber node):

        .. code-block:: bash

            rosrun my_cpp_package subscriber_node

- You should see the subscriber printing the messages published by the publisher.