Introduction.

In ROS2(Robot Operating System 2), a node is the smallest executable unit that performs computation. It is essentially a process that communicates with other nodes to exchange data (via topics, services, or actions) and perform tasks in a robotic system. ROS 2 publishers and subscribers are the basic communication mechanism between nodes using topics. They can be created by either using python or C++ language. In this blog we will be creating using C++.

Prerequisites

ROS installed. installing ros2
ROS2 workspace set up. setting up ros2 workspace
Package setup.
Creating a package — ROS 2 Documentation: Jazzy documentation
Basic Understanding of ROS concepts(nodes, topic, messages).
Beginner: CLI tools — ROS 2 Documentation: Jazzy documentation

Publisher-Subscriber Node in C++

1. Create a ROS 2 Package

cd ~ros2_ws/src
ros2 pkg create --build-type ament_cmake cpp_pubsub --dependencies rclcpp std_msgs

This creates a new C++ package named cpp_pubsub.

2. Add Publisher and Subscriber code

Navigate into the package:

cd ~/ros2_ws/src/cpp_pubsub/src

Create the following files in the src/ directory:

src/talker.cpp

Key in the code below in the opened file above:

#include <chrono>   // For time intervals like 500ms
#include <memory>  // For shared_ptr
#include "rclcpp/rclcpp.hpp"  // Core ROS 2 C++ client library
#include "std_msgs/msg/string.hpp"  // Message type used in the publisher
using namespace std::chrono_literals;  // Enables use of 500ms directly
class Talker : public rclcpp::Node {
public:
    Talker() : Node("talker_node"), count_(0) {
        publisher_ = this->create_publisher<std_msgs::msg::String>("chatter", 10);
        timer_ = this->create_wall_timer(500ms, std::bind(&Talker::publish_message, this));
    }
private:
    void publish_message() {
        auto message = std_msgs::msg::String();
        message.data = "Hello ROS2! Count: " + std::to_string(count_++);
        RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
        publisher_->publish(message);
    }
    rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
    rclcpp::TimerBase::SharedPtr timer_;
    int count_;
};
int main(int argc, char * argv[]) {
    rclcpp::init(argc, argv);  // Initialize ROS
    rclcpp::spin(std::make_shared<Talker>());   // Run node
    rclcpp::shutdown();  // Clean up
    return 0;
}

src/listener.cpp

Key in the code below in the opened file above:

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
class Listener : public rclcpp::Node {
public:
    Listener() : Node("listener_node") {
        subscription_ = this->create_subscription<std_msgs::msg::String>(
            "chatter", 10,
            [this](const std_msgs::msg::String::SharedPtr msg) {
                RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
            });
    }
private:
    rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};
int main(int argc, char * argv[]) {
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<Listener>());
    rclcpp::shutdown();
    return 0;
}

3. Update CMakeLists.txt

Update CMakeLists.txt and add to make:

cmake_minimum_required(VERSION 3.8)
project(cpp_pubsub)
if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()
# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
add_executable(talker src/talker.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
add_executable(listener src/listener.cpp)
ament_target_dependencies(listener rclcpp std_msgs)
install(TARGETS
  talker
  listener
  DESTINATION lib/${PROJECT_NAME})
if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  set(ament_cmake_copyright_FOUND TRUE)
  set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()
ament_package()

4. Update package.xml

Update the code to have:

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>cpp_pubsub</name>
  <version>0.1.0</version>
  <description>TODO: Package description</description>
  <maintainer email="wambuacaleb253@gmail.com">caleb253</maintainer>
  <license>MIT</license>
  <buildtool_depend>ament_cmake</buildtool_depend>
  <depend>rclcpp</depend>
  <depend>std_msgs</depend>
//Test dependencies
  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>
  <export>
    <build_type>ament_cmake</build_type>
  </export>
</package>

Ensure to include the dependencies if you did not include when creating the package.

  <depend>rclcpp</depend>
  <depend>std_msgs</depend>

5. Build the Package.

From your workspace root:

cd ~/ros2_ws
colcon build --packages-select cpp_pubsub

Then source your setup file:

source install/setup.bash

6. Run the Publisher and Subscriber.

In two separate terminals:

Terminal 1 (Publisher):

source ~/ros2_ws/install/setup.bash
ros2 run cpp_pubsub talker

Terminal 2 (Subscriber):

source ~/ros2_ws/install/setup.bash
ros2 run cpp_pubsub listener

You should be able to generate the results as shown below:

To stop the runs you should press Ctrl+c

Common Errors in ROS 2 C++ Publisher/Subscriber Nodes

1. Build Fails: Missing Dependency

Error

fatal error: rclcpp/rclcpp.hpp: No such file or directory

Cause

You didn’t declare the dependency on rclcpp in CMakeLists.txt or package.xml.

Fix

In `CMakeLists.txt`:

find_package(rclcpp REQUIRED)
target_link_libraries(your_node_name rclcpp::rclcpp)

In `package.xml`:

<build_depend>rclcpp</build_depend>
<exec_depend>rclcpp</exec_depend>

2. Build Succeeds, but Node Crashes at Runtime

Symptom

Segmentation fault when running the node.

Common Causes

You forgot to initialize ROS before using the node.
You used a raw pointer without initializing it.
Missing rclcpp::spin(…) in main().

Fix

Always structure main() like this:

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

3. Topic Mismatch: No Messages Received

Causes

Topic names differ ("chatter" vs "chat").
Mismatched message types.
Publisher or subscriber not active when tested.

Fix

Double-check both:

// Publisher
this->create_publisher<std_msgs::msg::String>("chatter", 10);
// Subscriber
this->create_subscription<std_msgs::msg::String>("chatter", 10, callback);

Use ros2 topic list and ros2 topic info to inspect topic state.

4. CMake Errors: Target Not Found

Error

CMake Error: Target "talker" not found

Cause

You forgot to define the executable or link libraries.

Fix in `CMakeLists.txt`:

add_executable(talker src/talker.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
install(TARGETS talker DESTINATION lib/${PROJECT_NAME})

5. "Could not find package configuration file"

Error

Could not find a package configuration file provided by "rclcpp"

Cause

Your environment isn't sourced properly.

Fix

source /opt/ros/humble/setup.bash
source ~/ros2_ws/install/setup.bash

6. Nothing Is Published or Logged

Cause

rclcpp::TimerBase::SharedPtr not retained — it gets garbage-collected.
You forgot to create the timer or use rclcpp::spin().

Fix

In your class:

rclcpp::TimerBase::SharedPtr timer_;  // define in class
timer_ = this->create_wall_timer(
    std::chrono::seconds(1),
    std::bind(&Talker::timer_callback, this));

7. Missing `#include` Statements

Error

‘cout’ was not declared in this scope

Cause

You’re using standard C++ things (like std::cout) but forgot the includes.

Fix

#include <iostream>

Similarly, always include:

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

8. Publisher Not Publishing

Cause

You forgot to store the publisher as a class member.

Fix

rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
publisher_ = this->create_publisher<std_msgs::msg::String>("chatter", 10);

If you declare the publisher inside a function (local scope), it will go out of scope and stop working.

9. Wrong `ament_target_dependencies` Syntax

Error

Unknown CMake command: ament_target_dependencies

Cause

You didn’t call find_package() before using ament_target_dependencies.
Typos in dependency names.

Fix

`find_package(rclcpp REQUIRED) find_package(std_msgs REQUIRED) ament_target_dependencies(your_node rclcpp std_msgs)`

References

Writing a simple publisher and subscriber (C++) — ROS 2 Documentation: Jazzy  documentation

CREATING PUBLISHER AND SUBSCRIBER NODE USING C++

Introduction.

Prerequisites

Publisher-Subscriber Node in C++

1. Create a ROS 2 Package

2. Add Publisher and Subscriber code

3. Update CMakeLists.txt

4. Update package.xml

5. Build the Package.

6. Run the Publisher and Subscriber.

Terminal 1 (Publisher):

Terminal 2 (Subscriber):

Common Errors in ROS 2 C++ Publisher/Subscriber Nodes

1. Build Fails: Missing Dependency

Error

Cause

Fix

In CMakeLists.txt:

In package.xml:

2. Build Succeeds, but Node Crashes at Runtime

Symptom

Common Causes

Fix

3. Topic Mismatch: No Messages Received

Causes

Fix

4. CMake Errors: Target Not Found

Error

Cause

Fix in CMakeLists.txt:

5. "Could not find package configuration file"

Error

Cause

Fix

6. Nothing Is Published or Logged

Cause

Fix

7. Missing #include Statements

Error

Cause

Fix

8. Publisher Not Publishing

Cause

Fix

9. Wrong ament_target_dependencies Syntax

Error

Cause

Fix

find_package(rclcpp REQUIRED) find_package(std_msgs REQUIRED) ament_target_dependencies(your_node rclcpp std_msgs)

References

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In `CMakeLists.txt`:

In `package.xml`:

Fix in `CMakeLists.txt`:

7. Missing `#include` Statements

9. Wrong `ament_target_dependencies` Syntax

`find_package(rclcpp REQUIRED) find_package(std_msgs REQUIRED) ament_target_dependencies(your_node rclcpp std_msgs)`