What is ament_cmake job?

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the main role of ament_cmake is to provide CMake macros that will make your job easier when developing multiple packages that depend on each other. None of it is required to build a CMake based ROS package.

An example of such macro is ament_target_dependencies, it condenses the following cmake logic into a single call:

• target_compile_definitions()
• target_include_directories()
• target_link_libraries()
• setting link flags on the target

Another (the most?) important macro it provides is ament_package. This is what will handle the registration of your package in the ament index, generate the CMake config files for the project to be find_package'able, install the extra CMake config files you provide, install the package.xml of your package etc

You can find some information about ament_cmakein the section "Build type: ament_cmake" of https://design.ros2.org/articles/amen...