ROS2 vs ROS1 Packages

ROS2 build system is colcon. In ROS1, the build system is catkin.

What is the difference between ROS1 and ROS2 packages? In ROS1, you can mix C++ and Python nodes in a single package. In ROS2, as a standard practice, you either make a C++ package or a Python package. [Although as a non-standard practice, it is possible in ROS2 to create C++ and Python nodes in a single package]. This difference can be observed from two sides below.

CLI commands to create a package

To create a ROS1 package, the CLI command is:

cd ~/catkin_ws/src
catkin_create_pkg [PACKAGE_NAME] roscpp rospy [OTHER_DEPENDENCY_PACKAGES]

To create a C++ package in ROS2, the CLI command is:

cd ~/ros2_ws/src
ros2 pkg create [PACKAGE_NAME] --build-type ament_cmake --dependencies rclcpp std_msgs [OTHER_DEPENDENCY_PACKAGES] 

To create a Python package in ROS2, the CLI command is:

cd ~/ros2_ws/src
ros2 pkg create [PACKAGE_NAME] --build-type ament_python --dependencies rclpy std_msgs [OTHER_DEPENDENCY_PACKAGES]

Structure of the “src” folder in the overlay workspace

The “src” folder in a ROS1 workspace looks like this:

workspace_folder:
|--src
   |--package
      |--CMakeLists.txt
      |--package.xml
      |--src
         |--node_file_1.cpp
         |--node_file_2.cpp
      |--include
      |--script
         |--node_file_1.py
         |--node_file_2.py
      |--

In the workspace structure above, “package” contains both C++ and Python source codes. Typically, the C++ source codes are located inside the “src” and “include” folders, whereas the Python source codes are located inside the “script” folder. The package.xml and CMakeLists.txt files configure both the C++ and Python packages.

The “src” folder in a ROS2 workspace looks like this:

workspace_folder:
|--src
   |--package_1
      |--CMakeLists.txt
      |--package.xml
      |--src
         |--node_file.cpp
      |--include
      |--
   |--package_2
      |--setup.py
      |--setup.cfg
      |--package.xml
      |--package_2
         |--__init__.py
         |--node_file.py
      |--resource
         |--package_2
      |--test
         |--test_copyright.py
         |--test_flake8.py
         |--test_pep257.py

In the workspace structure above, package_1 is the source folder of a C++ package whereas package_2 is the source folder of a Python package. Each of the packages is either a C++ package or a Python package.