Using a custom message in C++ (ROS2 foxy)

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I have a project that I am working on where I am (so far) communicating with a microcontroller with one node and sending commands to that node from another node. I started out just using a string to send everything because it was the fastest way to get going but now I am trying to mend my ways and do things properly. It seems that the proper way is to create my own messages, even if I am just sending a single byte that might otherwise be covered by something like std_msgs::int32. That is, of course, unless I am sending data where there is already an established message type for that exact thing.

Anyway, I have followed along with a couple of online tutorials and both of them have allowed me to make a custom message type that I can then publish with using the command line. I can also receive them using the command line. So far so good. Now I want to incorporate this into my programs. I'm getting stuck there though. My understanding is that the build process of the message project should generate a .hpp file that I can use form my own code. It appears that this is how the std_msgs are generated. If I go into /opt/ros/foxy/include/std_msgs/msg/int16.hpp, it has "Generated from rosidl_generator_cpp/resource/idl.hpp.em" in it.

Given that I can publish and subscribe from the command line, I feel like I am 95% of the way through this but am missing something simple.

I am working on a Raspberry Pi 4B using Ubuntu 20.04 server and ROS2 foxy. I am not getting any errors when running colcon build.

If anyone could provide help, I would appreciate it.

Below is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)
project(jack_test)

# Default to C99
if(NOT CMAKE_C_STANDARD)
  set(CMAKE_C_STANDARD 99)
endif()

# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 14)
endif()

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)   # This was added because of the --dependencies when creating the package
find_package(std_msgs REQUIRED) # This was added because of the --dependencies when creating the package
find_package(builtin_interfaces REQUIRED) # ADDED
find_package(rosidl_default_generators REQUIRED) # ADDED



if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  # the following line skips the linter which checks for copyrights
  # uncomment the line when a copyright and license is not present in all source files
  #set(ament_cmake_copyright_FOUND TRUE)
  # the following line skips cpplint (only works in a git repo)
  # uncomment the line when this package is not in a git repo
  #set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()

rosidl_generate_interfaces(${PROJECT_NAME}_msg "msg/SampleMessage.msg" DEPENDENCIES builtin_interfaces LIBRARY_NAME ${PROJECT_NAME})

ament_package()

Below is my 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>jack_test</name>
  <version>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="fakeemail@nowhere.com">ubuntu< ...