Question

I’m currently trying to put part of a complex program into a dynamic library. This part consists of some classes that are also wrapped with boost python into a module to be embedded again. Here is a simplified version of the dll source.

Hello.cpp:

#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>
#include <boost/python.hpp>
#include <Foo.h>

using namespace boost::python;

typedef boost::shared_ptr<Hello> hello_ptr;

BOOST_PYTHON_MODULE(Hello)
{
    class_<Hello, hello_ptr>("Hello")
        .def("say_hello", &Hello::say_hello)
    ;
};


void Hello::say_hello(){
    cout << "Hello World!" << endl;
}

void World::foo(){
    Py_Initialize();

    try {
        PyRun_SimpleString(
            "hello = None\n"
            "\n"
            "def setup(hello_from_cxx):\n"
            "    print 'setup called with', hello_from_cxx\n"
            "    global hello\n"
            "    hello = a_foo_from_cxx\n"
            "\n"
            "def run():\n"
            "    hello.say_hello()\n"
            "\n"
            "print 'main module loaded'\n"
        );
        //initialize eviroment
        initHello();

        hello_ptr hello = boost::make_shared<Hello>();

        object main = object(handle<>(borrowed(
            PyImport_AddModule("__main__")
        )));

        // pass the reference to hello into python:
        object setup_func = main.attr("setup");
        setup_func(hello);

        // now run the python 'main' function
        object run_func = main.attr("run");
        run_func();
    }
    catch (error_already_set) {
        PyErr_Print();
    }
}

Hello.h

#ifndef HELLO_H_INCLUDED
#define HELLO_H_INCLUDED

#include <iostream>
#include <boost/python.hpp>

using namespace std;

class Hello{
public:
    void say_hello();
};

class World{
public:
    void foo();
};

#endif // HELLO_H_INCLUDED

In the main function of the application I then create a instance of the class that embeds python but it gives an undefined reference to World::foo() even though the function is defined in the dynamic library libFoo.dll which is also linked against by the main application.

main.cpp:

#include <iostream>
#include <Hello.h>

using namespace std;

int main(){
    cout << "Test" << endl;

    World world;

    world.foo();
}

Console:

undefined reference to `World::foo()'

I’m using Code::Blocks with MinGW.

This problem has kept me awake some days now and I can’t seem to find a way to solve it. Hope you can help me.

Thanks in advance.

UPDATE:

I now tried to slove this task using the normal Python C API. There the step of defining the Python_Module (BOOST_PYTHON_MODULE()) to which this error is definitely connected, is done by defining a function like:

PyMODINIT_FUNC initHello(void){
...
}

Now writing infront of this

#define PyMODINIT_FUNC void

solves the error. Does anyone now how the boost.python library defines the BOOST_PYTHON_MODULE function? Maybe there is this kind of solution as well? I couldn’t find any PyMODINIT_FUNC stuff in the source files.

Answer 1

Found the solution myself.

It’s like I had already guessed. What is needed is to define BOOST_PYTHON_MODULE_INIT(name) like this.

#   define BOOST_PYTHON_MODULE_INIT(name)                               \
  void BOOST_PP_CAT(init_module_,name)();                               \
extern "C" __attribute__ ((__visibility__("default"))) _BOOST_PYTHON_MODULE_INIT(name)

Actually I found this in one of the header files with some #ifdef infront but it seems that they’re not working correctly. At least not for me.