[question] PkgConfig.fill_cpp_info() Overwrite Existing self.cpp_info

[tianyi-lu-ocean]

What is your question?

I have a producer package which uses a system library libcamera. To achieve that, I added the following lines in its CMakeLists.txt,

find_package(PkgConfig)
# Then try to find system libcamera to find the corresponding cameras
pkg_check_modules(LIBCAMERA REQUIRED IMPORTED_TARGET libcamera)
message(STATUS "libcamera library found:")
message(STATUS "    version: ${LIBCAMERA_VERSION}")
message(STATUS "    libraries: ${LIBCAMERA_LINK_LIBRARIES}")
message(STATUS "    include path: ${LIBCAMERA_INCLUDE_DIRS}")

however, in the consumer package's CMakeLists.txt, I have to do the same thing which is quite annoying. I know I can leverage find_dependency() in CMake, but as I rely on CMakeDeps generator, I don't know how to add that into the auto generated <PackageName>Config.cmake file.

After some investigation, I found a recipe tool PkgConfig in conan.tools.gnu category. So, I think I can do something in producer's recipe, like,

def package_info(self);
    pkg_config = PkgConfig(self, 'libcamera')
    pkg_config.fill_cpp_info(self.cpp_info, is_system = True)

and it probably will propagate the include path and library path into consumer. in which case I don't have to duplicate those CMake stuffs in consumer's CMakeLists.txt.

However, it doesn't work, because the PkgConfig.fill_cpp_info() will overwrite the self.cpp_info. I also looked into the source code of PkgConfig, it applies assignment operation for those lists in cpp_info, please see the line here, I guess line 116 could be a potential bug, because it tries to assign a different list to the cpp_info.cxxflags, or maybe the pkg-config does not support cxxflags? Another issue would be the comment of system_libs parameter in fill_cpp_info() which also affects the conan documentation, it seems like just copying the comments of 'is_system`.

Anyway, my questions are,

• Is this an intentional implementation that will overwrite the incoming cpp_info anyway?
• Is that possible to just append those lists into the existing lists in cpp_info?

Incidentally, I tried to pass a temporary cpp_info into it, but due to Python passing by reference feature, it will still overwrite the original self.cpp_info. Besides that, I also tried to copy.deepcopy(), but for some reason, it gives me a TypeError exception. So, my current solution is,

• copy every lists in self.cpp_info into some temporary lists
• invoke fill_cpp_info(self.cpp_info)
• append those copied temporary lists back to the self.cpp_info

I don't think this is a decent solution, I would expect to do those in fill_cpp_info().

Have you read the CONTRIBUTING guide?

• I've read the CONTRIBUTING guide

[memsharded]

Hi @tianyi-lu-ocean

Thanks for your question.

I think you might use a different approach, like first getting the cpp_info object of PkgConfig, then merging that information with the existing one, instead of overwriting it.

Something like this (just the idea, not tested):

def package_info(self):
    cpp_info = CppInfo()
    pkg_config.fill_cpp_info(cpp_info, is_system=False, system_libs=["m"])

    self.cpp_info.merge(cpp_info)

The merge is documented in https://docs.conan.io/2/reference/tools/cpp_info.html

You might even invert the order for different priority:

cpp_info = CppInfo()
pkg_config.fill_cpp_info(cpp_info, is_system=False, system_libs=["m"])
cpp_info.merge(self.cpp_info)
self.cpp_info = cpp_info

Please try that and let us know. Thanks!