Creating C++ Nodes (for ament_cmake)

The C++ binary block for ament_cmake

Create the package

Warning

We’ll skip using the --node-name option to create the Node template, because, currently, it generates a Node and a CMakeLists.txt different from my advice.

cd ~/ros2_tutorial_workspace/src
ros2 pkg create cpp_package_with_a_node \
--build-type ament_cmake \
--dependencies rclcpp

which outputs

Package-related sources

In this step, we’ll work on these.

cpp_package_with_a_node
├── CMakeLists.txt
├── include
│   └── cpp_package_with_a_node
│       └── .placeholder
├── package.xml
└── src
    ├── print_forever_node.cpp
    ├── print_forever_node.hpp
    └── print_forever_node_main.cpp

The files already exist, we just need to modify them as follows

package.xml

The package.xml works the same way as in ament_python, with the exception of the two lines about ament_cmake shown below.

package.xml

<?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_package_with_a_node</name>
  <version>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="murilomarinho@ieee.org">murilo</maintainer>
  <license>TODO: License declaration</license>

  <buildtool_depend>ament_cmake</buildtool_depend>

  <depend>rclcpp</depend>

  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>

  <export>
    <build_type>ament_cmake</build_type>
  </export>
</package>

CMakeLists.txt

A one-size-fits-most solution is shown below. For each new Node we add a block to the CMakeLists.txt with the following format.

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(cpp_package_with_a_node)

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)

############################
# CPP Binary Block [BEGIN] #
# vvvvvvvvvvvvvvvvvvvvvvvv #
# https://ros2-tutorial.readthedocs.io/en/latest/
# While we cant use blocks https://cmake.org/cmake/help/latest/command/block.html#command:block
# we use set--unset
set(RCLCPP_LOCAL_BINARY_NAME print_forever_node)

add_executable(${RCLCPP_LOCAL_BINARY_NAME}
    src/print_forever_node_main.cpp
    src/print_forever_node.cpp

    )

ament_target_dependencies(${RCLCPP_LOCAL_BINARY_NAME}
    rclcpp

    )

target_link_libraries(${RCLCPP_LOCAL_BINARY_NAME}

    )

target_include_directories(${RCLCPP_LOCAL_BINARY_NAME} PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>)

target_compile_features(${RCLCPP_LOCAL_BINARY_NAME} PUBLIC c_std_99 cxx_std_17)

install(TARGETS ${RCLCPP_LOCAL_BINARY_NAME}
    DESTINATION lib/${PROJECT_NAME})

unset(RCLCPP_LOCAL_BINARY_NAME)
# ^^^^^^^^^^^^^^^^^^^^^^ #
# CPP Binary Block [END] #
##########################

if(BUILD_TESTING)
    find_package(ament_lint_auto REQUIRED)
    # the following line skips the linter which checks for copyrights
    # comment the line when a copyright and license is added to all source files
    set(ament_cmake_copyright_FOUND TRUE)
    # the following line skips cpplint (only works in a git repo)
    # comment the line when this package is in a git repo and when
    # a copyright and license is added to all source files
    set(ament_cmake_cpplint_FOUND TRUE)
    ament_lint_auto_find_test_dependencies()
endif()

ament_package()

Making C++ ROS2 Nodes

(Murilo’s) rclcpp best practices

For each new C++ Node, we make three files following the style below.

For a Node called print_forever_node we have

src/print_forever_node.hpp with the Node’s class definition. In general, this is not exported to other packages, so it should not be in the package’s include folder.
src/print_forever_node.cpp with the Node’s class implementation.
src/print_forever_node_main.cpp with the Node’s main function implementation.

In this step, we’ll work on these.

cpp_package_with_a_node
├── CMakeLists.txt
├── include
│   └── cpp_package_with_a_node
│       └── .placeholder
├── package.xml
└── src
    ├── print_forever_node.cpp
    ├── print_forever_node.hpp
    └── print_forever_node_main.cpp

These files do not exists, so we’ll create them.

folder

cd ~/ros2_tutorial_workspace/src/cpp_package_with_a_node
mkdir src

src/…_node.hpp

Similar to what we did in Python, we inherit from rclcpp::Node. Whatever is different is owing to differences in languages.

print_forever_node.hpp

#pragma once

#include <memory>
#include <rclcpp/rclcpp.hpp>

/**
 * @brief A ROS2 Node that prints to the console periodically, but in C++.
 */
class PrintForeverNode: public rclcpp::Node
{
private:
    double timer_period_;
    int print_count_;
    //also equivalent to rclcpp::TimerBase::SharedPtr
    std::shared_ptr<rclcpp::TimerBase> timer_;

    void _timer_callback();
public:
    PrintForeverNode();

};

src/…_node.cpp

The implementation has nothing special, just don’t forget to initialize the parent class, rclcpp::Node, with the name of the node.

print_forever_node.cpp

#include "print_forever_node.hpp"

/**
 * @brief PrintForeverNode::PrintForeverNode Default constructor.
 */
PrintForeverNode::PrintForeverNode():
    rclcpp::Node("print_forever_cpp"),
    timer_period_(0.5),
    print_count_(0)
{
    //(Smart) pointers at the one thing that it doesn't matter much if they are not initialized in the member initializer list
    //and this is a bit more readable.
    timer_ = create_wall_timer(
                std::chrono::milliseconds(long(timer_period_*1e3)),
                std::bind(&PrintForeverNode::_timer_callback, this) //Note here the use of std::bind to build a single argument
                );
}

/**
 * @brief PrintForeverNode::_timer_callback periodically prints class info using RCLCPP_INFO.
 */
void PrintForeverNode::_timer_callback()
{
    RCLCPP_INFO_STREAM(get_logger(),
                       std::string("Printed ") +
                       std::to_string(print_count_) +
                       std::string(" times.")
                       );
    print_count_++;
}

src/…_main.cpp

Given that we are using rclcpp::spin(), there is nothing special here either. Just remember to not mess up the std::make_shared and always use perfect forwarding. See Perfect forwarding. The rclcpp::spin() handles the SIGINT when we, for example, press CTRL+C on the terminal. It is not perfect, but it does the trick for simple nodes like this one.

print_forever_node_main.cpp

#include <rclcpp/rclcpp.hpp>

#include "print_forever_node.hpp"

int main(int argc, char** argv)
{
    rclcpp::init(argc,argv);

    try
    {
        auto node = std::make_shared<PrintForeverNode>();

        rclcpp::spin(node);
    }
    catch (const std::exception& e)
    {
        std::cerr << std::string("::Exception::") << e.what();
    }

    return 0;
}

Add a `.placeholder` if your `include/<PACKAGE_NAME>` is empty

Warning

If you don’t do this and add this package as a git repository without any files on the include/, CMake might return with an error when trying to compile your package.

cpp_package_with_a_node
├── CMakeLists.txt
├── include
│   └── cpp_package_with_a_node
│       └── .placeholder
├── package.xml
└── src
    ├── print_forever_node.cpp
    ├── print_forever_node.hpp
    └── print_forever_node_main.cpp

Empty directories will not be tracked by git. A file has to be added to the index. We can create an empty file in the include folder as follows

cd ~/ros2_tutorial_workspace/src/cpp_package_with_a_node/src
touch include/cpp_package_with_a_node/.placeholder

Running a C++ Node

As simple as it has always been, see Running a node (ros2 run).

ros2 run cpp_package_with_a_node print_forever_node

which returns

[INFO] [1688620414.406930812] [print_forever_node]: Printed 0 times.
[INFO] [1688620414.906890884] [print_forever_node]: Printed 1 times.
[INFO] [1688620415.406907619] [print_forever_node]: Printed 2 times.
[INFO] [1688620415.906881003] [print_forever_node]: Printed 3 times.
[INFO] [1688620416.406900108] [print_forever_node]: Printed 4 times.
[INFO] [1688620416.906886691] [print_forever_node]: Printed 5 times.
[INFO] [1688620417.406881803] [print_forever_node]: Printed 6 times.
[INFO] [1688620417.906858551] [print_forever_node]: Printed 7 times.
[INFO] [1688620418.406894922] [print_forever_node]: Printed 8 times.

and we’ll use CTRL+C to stop the node, resulting in

[INFO] [1688620418.725674401] [rclcpp]: signal_handler(signum=2)