Qt binding to python - undefined symbol

[hanusek]

Hello! I have a problem with binding C++/Qt code to Python. When I import my library in Python: ImportError: my_lib_python_api.so: undefined symbol: _ZTV14FakeAuthThread I notice it's happening when I put Q_OBJECT in my class (FakeAuthThread). How to fix it?

My example lib (file: PythonClient.cpp)

class FakeAuthThread
{
    Q_OBJECT // it's problem

public:
    FakeAuthThread() = default;

    void stop() {}

    void run() {}
};

class AuthThreadWrapper
{
public:
    AuthThreadWrapper(){};

    void init()
    {
        _thread = new FakeAuthThread();
    }

private:
    FakeAuthThread *_thread{nullptr};
};

BOOST_PYTHON_MODULE(my_lib_python_api)
{
    Py_Initialize();

    class_<AuthThreadWrapper, boost::noncopyable>("AuthThreadWrapper")
        .def("init", &AuthThreadWrapper::init);
}

CMakeLists.txt

 ..
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 set(CMAKE_VERBOSE_MAKEFILE ON)
 set(CMAKE_VERBOSE_BUILD ON)
 set(CMAKE_COMPILE_WARNING_AS_ERROR ON)
 set(CMAKE_ENABLE_EXPORTS ON)

 find_package(Qt5 COMPONENTS REQUIRED Core Widgets)
 find_package(Boost REQUIRED COMPONENTS python)
 find_package(PythonLibs ${PY_VERSION} REQUIRED)
 find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
 include_directories(${Python3_INCLUDE_DIRS})

 add_library(my_lib_python_api SHARED python/PythonClient.cpp)
 target_link_libraries(my_lib_python_api PUBLIC Boost::python Python3::Python Qt5::Core)
 set_target_properties(my_lib_python_api PROPERTIES PREFIX "")

ldd:

  linux-vdso.so.1 (0x00007ffddc96c000)
        libpython3.10.so.1.0 => /lib/x86_64-linux-gnu/libpython3.10.so.1.0 (0x00007f9472f78000)
        libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f9472d4c000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f9472c63000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9472c43000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f9472a1a000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f9474360000)
        libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f94729e9000)
        libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f94729cd000)
undefined symbol: _ZTV14FakeAuthThread  (./my_lib_python_api.so)
undefined symbol: _ZTI14FakeAuthThread  (./my_lib_python_api.so)

 $ readelf -Ws --dyn-syms ./my_lib_python_api.so | grep "FakeAuthThread"
   162: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _ZTV14FakeAuthThread
   270: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _ZTI14FakeAuthThread

Qt version: 5.15.3 Boost version: 1.77.0 Python3_VERSION: 3.10.12 Python3_INCLUDE_DIRS: /usr/include/python3.10 Python3_LIBRARIES: /usr/lib/x86_64-linux-gnu/libpython3.10.so

[stefanseefeld]

Hi Michal,

I suspect the problem has nothing to do with Boost.Python, but rather with your CMake logic: As you are using the QOBJECT macro in your FakeAuthThread class, you need to process that code with Qt's moc to generate (and subsequently compile) additional code. See https://cmake.org/cmake/help/latest/prop_tgt/AUTOMOC.html

[hanusek]

I added:

  set(CMAKE_AUTOMOC ON)
  set_target_properties(my_lib_python_api PROPERTIES AUTOMOC TRUE)

And it works.