ament_target_dependencies not linking an upstream ament package?

Question

Background

• Upstream pkg (called rclcpp_sample) provides a library-a.
• Downstream pkg (called rclcpp_child_sample) provides an executable that depends on library-a.
• Both are ament-packages (i.e. written within ament framework), to be built with colcon.

To avoid a persistent linker error (very similar to robotics.stackexchange.com#23995 but I wouldn't say the same), the solution I came up with was to target_link_libraries the target that the upstream pkg exports.

target_link_libraries(${app_derived} rclcpp_sample::rclcpp_sample)

Questions

• Q1. Why ament_target_dependencies can't link to a resource in another ament package?

  Common error I got was:

  # colcon build --packages-select rclcpp_sample rclcpp_child_sample
  Starting >>> rclcpp_sample
  Finished <<< rclcpp_sample [1.46s]                    
  Starting >>> rclcpp_child_sample
  --- stderr: rclcpp_child_sample                              
  /usr/bin/ld: CMakeFiles/app_derived.dir/src/exec.cpp.o: in function `void __gnu_cxx::new_allocator<rclcpp_sample::DerivedModule>::construct<rclcpp_sample::DerivedModule, char const (&) [9]>(rclcpp_sample::DerivedModule*, char const (&) [9])':
  exec.cpp:(.text._ZN9__gnu_cxx13new_allocatorIN13rclcpp_sample13DerivedModuleEE9constructIS2_JRA9_KcEEEvPT_DpOT0_[_ZN9__gnu_cxx13new_allocatorIN13rclcpp_sample13DerivedModuleEE9constructIS2_JRA9_KcEEEvPT_DpOT0_]+0x79): undefined reference to `rclcpp_sample::DerivedModule::DerivedModule(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
  collect2: error: ld returned 1 exit status
  make[2]: *** [CMakeFiles/app_derived.dir/build.make:132: app_derived] Error 1
  make[1]: *** [CMakeFiles/Makefile2:78: CMakeFiles/app_derived.dir/all] Error 2
  make: *** [Makefile:141: all] Error 2
  ---
  Failed   <<< rclcpp_child_sample [2.67s, exited with code 2]
  

• Q2. Why the exported macro is duplicating the element as rclcpp_sample::rclcpp_sample? rclcpp_sample is the namespace I use but the duplication of it is never defined anywhere in the upstream.

More background

Full code of 2 packages can be seen in prooving_grounds!2 if that's quicker.

Upstream pkg

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(rclcpp_sample)

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

# BEGIN: From ROS2 tutorial https://docs.ros.org/en/galactic/Tutorials/Beginner-Client-Libraries/Writing-A-Simple-Cpp-Publisher-And-Subscriber.html
if(NOT CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 17)
endif()
if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()
# END From ROS2 tutorial

set(THIS_PACKAGE_INCLUDE_DEPENDS
  ament_cmake
  rclcpp 
)
foreach(Dependency IN ITEMS ${THIS_PACKAGE_INCLUDE_DEPENDS})
  find_package(${Dependency} REQUIRED)
endforeach()

add_library(${PROJECT_NAME} SHARED
  src/impl.cpp
)
target_include_directories(
  ${PROJECT_NAME}
  PUBLIC
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>)
#target_link_libraries(${PROJECT_NAME} ${THIS_PACKAGE_INCLUDE_DEPENDS})    
ament_target_dependencies(${PROJECT_NAME} ${THIS_PACKAGE_INCLUDE_DEPENDS})
ament_export_include_directories(include)
ament_export_targets(${PROJECT_NAME}Targets HAS_LIBRARY_TARGET)

install(DIRECTORY include/
  DESTINATION include
)
install(
  TARGETS ${PROJECT_NAME}
  EXPORT ${PROJECT_NAME}Targets
  INCLUDES DESTINATION include
  RUNTIME DESTINATION bin
  LIBRARY DESTINATION lib
  ARCHIVE DESTINATION lib
)

ament_package()

base.hpp (defines the parent of another class)

#ifndef RCLCPP_SAMPLE__BASE_HPP_
#define RCLCPP_SAMPLE__BASE_HPP_

#include <ament_index_cpp/get_package_share_directory.hpp>
#include <unistd.h>
#include <rclcpp/rclcpp.hpp>
#include <string>

namespace rclcpp_sample
{

class BaseModules : public rclcpp::Node
{
public:
  BaseModules(const std::string& nodename);
  virtual ~BaseModules();

  virtual bool func_to_be_overridden();
};
}  // namespace rclcpp_sample

#endif  //RCLCPP_SAMPLE__BASE_HPP_

child1.hpp

#ifndef RCLCPP_SAMPLE__CHILD1_HPP_
#define RCLCPP_SAMPLE__CHILD1_HPP_

#include <string>

#include <rclcpp_sample/base.hpp>

namespace rclcpp_sample
{

class DerivedModule : public BaseModules
{
public:
  explicit DerivedModule(const std::string& nodename);
  virtual ~DerivedModule();

  virtual bool func_to_be_overridden();
};
}  // namespace rclcpp_sample

#endif  // RCLCPP_SAMPLE__CHILD1_HPP_

impl.cpp

#include <rclcpp_sample/base.hpp>
#include <rclcpp_sample/child1.hpp>

using namespace rclcpp_sample;

BaseModules::BaseModules(const std::string& nodename) :
    rclcpp::Node(nodename)
{
  RCLCPP_INFO(this->get_logger(), "Initialization.");
}

BaseModules::~BaseModules()
{
  RCLCPP_INFO(this->get_logger(), "Deconstruction.");
}

bool BaseModules::func_to_be_overridden()
{
  return false;
}

DerivedModule::DerivedModule(const std::string& nodename) :
    BaseModules(nodename)
{
  RCLCPP_INFO(get_logger(), "Construtor of DerivedModule class.");
}

DerivedModule::~DerivedModule()
{
}

bool DerivedModule::func_to_be_overridden()
{
  return true;
}

Downstream pkg

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(rclcpp_child_sample)

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

set(THIS_PACKAGE_INCLUDE_DEPENDS
  ament_cmake
  rclcpp
  rclcpp_sample
)
foreach(dep IN ITEMS ${THIS_PACKAGE_INCLUDE_DEPENDS})
  find_package(${dep} REQUIRED)
endforeach()

set(app_derived app_derived)
add_executable(${app_derived} src/exec.cpp)
target_link_libraries(${app_derived} rclcpp_sample::rclcpp_sample)
ament_target_dependencies(${app_derived} ${THIS_PACKAGE_INCLUDE_DEPENDS})
install(TARGETS ${app_derived} RUNTIME DESTINATION lib/${PROJECT_NAME})

ament_package()

exec.cpp

#include <rclcpp/rclcpp.hpp>
#include <string>

#include <rclcpp_sample/base.hpp>
#include <rclcpp_sample/child1.hpp>

int main(int argc, char *argv[])
{
  rclcpp::init(argc, argv);
  auto app = std::make_shared<rclcpp_sample::DerivedModule>("app_execo");
  rclcpp::executors::MultiThreadedExecutor executor;
  executor.add_node(app);
  std::thread([&executor]() { executor.spin(); }).detach();
  RCLCPP_INFO(rclcpp::get_logger("exec_rclcpp_sample"),
              "func_to_be_overridden: %d", app->func_to_be_overridden());
  rclcpp::shutdown();
}

Answer 1

First and foremost, I'm not able to reproduce your linking error on Fedora using either Rolling or Humble. I dropped the target_link_libraries call that you referenced in rclcpp_child_sample, but librclcpp_sample.so still appears on the link line (as expected) and the operation succeeds.

Regarding the namespacing, the convention I've seen when namespacing in CMake is <PROJECT_NAME>::<TARGET_NAME>. This is completely disjoint from any C++ namespacing your libraries might have. In your case, the project name and the single target produced in it share the same name so it may seem redundant in such a simple case, but it does make sense for more complex projects.