using ament_cmake to build and use a shared library

edit

I am building a shared library for use in another package. I'm using ROS2 dashing.

I followed the instructions of the ament_cmake user documentation, but the client project could not find my library's header files unless I added the ament_export_include_directories(include) and ament_export_libraries(my_library) functions, which the documentation says are superfluous.

Here is the CMakeList.txt for my library:

cmake_minimum_required(VERSION 3.5)
project(macaroons)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

find_package(ament_cmake REQUIRED)

add_library(macaroons SHARED
            src/macaroons.cpp
            )

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

ament_export_interfaces(export_macaroons HAS_LIBRARY_TARGET)   # <-- documentation prepends export_

install(
  DIRECTORY include/
  DESTINATION include
)

install(
  TARGETS macaroons
  EXPORT export_macaroons     # <-- documentation prepends export_
  ARCHIVE DESTINATION lib
  LIBRARY DESTINATION lib
  RUNTIME DESTINATION bin
  INCLUDES DESTINATION include
)

ament_export_include_directories(include)     # <-- superfluous?
ament_export_libraries(macaroons)             # <-- superfluous?

ament_package()

This places the library in <prefix>/install/macaroons/lib/libmacaroons.so and the API header files in <prefix>/install/macaroons/include/macaroons/macaroons.hpp, which is where I would expect them to be.

If I do not include the "superfluous" commands, then when try to find_packag() and use ament_target_dependencies(), I get fatal error: macaroons/macaroons.hpp: No such file or directory. If I do include them, it builds just fine.

Here is the client project's CMakeList.txt file:

cmake_minimum_required(VERSION 3.5)
project(macaroon_demo)

# 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_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(rclcpp_components REQUIRED)
find_package(std_msgs REQUIRED)
find_package(macaroons REQUIRED)

add_executable(macaroon_demo
              src/macaroon_demo.cpp
              src/talker_node.cpp
              src/listener_node.cpp)
target_include_directories(macaroon_demo PRIVATE
  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
  $<INSTALL_INTERFACE:include>)
ament_target_dependencies(macaroon_demo rclcpp rclcpp_components std_msgs macaroons)

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

ament_package()

I'm grateful that it builds, but I would also be grateful for an explanation of where my library's CMakeList.txt file is incorrect.