How to write header file for ROS 2 publisher

Question

Hello,

I am relatively new to ROS 2. I have worked on ROS and can make header files for my class. I am trying to replicate the same thing for ROS 2. I am following the Pub Sub tutorial on ROS 2 documentation. However, in that they have defined the class and the main function inside one .cpp file. I want to define the class in a .h file, make a corresponding .cpp file of the class and call the entire thing in a main.cpp file

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Originally posted by cjoshi17 on ROS Answers with karma: 28 on 2021-06-11

Post score: 1

Answer 1

It can be done, you just need to build the files correctly. Taking the minimum publisher as an example, you can break it up into different files:

minimal_publisher.hpp

#include <chrono>
#include <functional>
#include <memory>
#include <string>

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"


class MinimalPublisher : public rclcpp::Node
{
public:
  MinimalPublisher();

 private:
  void timer_callback();
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
  size_t count_;
};

minimal_publisher.cpp

#include "minimal_publisher.hpp"


MinimalPublisher::MinimalPublisher ()
: Node("minimal_publisher"), count_(0)
{  
    publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
    timer_ = this->create_wall_timer(
      500ms, std::bind(&MinimalPublisher::timer_callback, this));
}

MinimalPublisher::timer_callback()
{
    auto message = std_msgs::msg::String();
    message.data = "Hello, world! " + std::to_string(count_++);
    RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
    publisher_->publish(message);
 }

minimal_publisher_node.cpp

#include "minimal_publisher.hpp"

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalPublisher>());
  rclcpp::shutdown();
  return 0;
}

Now you just need to be able to build it correctly. If you don't care exactly, you can wrap it all into one add_executable call in the CMakeLists.txt. Or, you can build them minimal_publisher.hpp and .cpp as a library and then the minimal_publisher_node.cpp as an executable and link the library.

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Originally posted by 404RobotNotFound with karma: 331 on 2021-06-11

This answer was ACCEPTED on the original site

Post score: 4

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Original comments

Comment by cjoshi17 on 2021-06-11:
Hi,

Thanks for solving my problem. I had defined my .h and corresponding .cpp file like you have mentioned in the answer. The step I was doing wrong was in the CMakeLists.txt. I defined an executable for my publisher.cpp, subscriber.cpp and main.cpp instead of having it all defined in one executable like this

Wrong way:

add_executable(talker src/publisher.cpp) ament_target_dependencies(talker rclcpp std_msgs)

add_executable(listener src/subscriber.cpp) ament_target_dependencies(listener rclcpp std_msgs)

add_executable(main_node src/main.cpp) ament_target_dependencies(main_node rclcpp std_msgs)

Correct way:

add_executable(main_node src/main.cpp src/publisher.cpp src/subscriber.cpp) ament_target_dependencies(main_node rclcpp std_msgs)

Thanks again for solving my problem. I have been coding in python for a few months now and hence forgotten about the whole CMake definitions for C++.