ros2，功能包下使用自定义的msg，msg文件在定义在当前功能包下

1.创建功能包，参考ros官方方式Writing a simple publisher and subscriber (C++) — ROS 2 Documentation: Humble documentation

ros2 pkg create --build-type ament_cmake --license Apache-2.0 cpp_pubsub

2.创建msg

在cpp_pubsub功能包下创建msg文件夹  ros2_ws/cpp_pubsub/msg

mkdir msg

在msg文件夹下创建.msg文件   ros2_ws/cpp_pubsub/msg/Demo.msg

touch Demo.msg 

Demo.msg定义如下

int64 num
std_msgs/String str

3.修改cpp_pubsub功能包下的package.xml文件

xml文件内添加 

<buildtool_depend>rosidl_default_generators</buildtool_depend>
<exec_depend>rosidl_default_runtime</exec_depend>
<member_of_group>rosidl_interface_packages</member_of_group>

4.CmakeLists.txt添加

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Demo.msg"
  DEPENDENCIES std_msgs
)
#导出运行时依赖 
ament_export_dependencies(rosidl_default_runtime)

5.修改功能CmakeLists.txt 要编译的 talker listener node节点的依赖

add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp)


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

rosidl_get_typesupport_target(cpp_typesupport_target
  ${PROJECT_NAME} rosidl_typesupport_cpp)
 
target_link_libraries(talker "${cpp_typesupport_target}")
target_link_libraries(listener "${cpp_typesupport_target}")
 
ament_export_dependencies(rosidl_default_runtime)

6.完整的package.xml和CmakeLists.txt如下

#xml文件
<?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>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="">nvidia</maintainer>
  <license>TODO: License declaration</license>

  <buildtool_depend>ament_cmake</buildtool_depend>
  <depend>rclcpp</depend>
  <depend>std_msgs</depend>

  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>
  
  <buildtool_depend>rosidl_default_generators</buildtool_depend>
  <exec_depend>rosidl_default_runtime</exec_depend>
  <member_of_group>rosidl_interface_packages</member_of_group>

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

#CMakeLists.txt文件
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()

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
  DEPENDENCIES std_msgs
)

add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp)


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

rosidl_get_typesupport_target(cpp_typesupport_target
  ${PROJECT_NAME} rosidl_typesupport_cpp)
 
target_link_libraries(talker "${cpp_typesupport_target}")
target_link_libraries(listener "${cpp_typesupport_target}")
 
ament_export_dependencies(rosidl_default_runtime)

ament_package()

7.publisher_member_function.cpp文件代码修改

void timer_callback()
  {
    auto message = cpp_pubsub::msg::Num();
    message.num = (int64_t)count_++;
    message.str.data = "Hello, world!";
    RCLCPP_INFO(this->get_logger(), "Publishing: %s %ld", message.str.data.c_str(), message.num);
    publisher_->publish(message);
  }

8.subscriber_member_function.cpp文件修改

void topic_callback(const cpp_pubsub::msg::Num & msg) const
  {
    RCLCPP_INFO(this->get_logger(), "I heard: %s %ld", msg.str.data.c_str(), msg.num);
  }