Question

I’m trying to expose one of my classes to python using boost’s python module. I have a class called StatusEffect that I’d like to expose to python. To do so, I’m creating this in the StatusEffect.h file after the class definition of StatusEffect

BOOST_PYTHON_MODULE(StatusEffect)
{
    boost::python::class_<StatusEffect>("StatusEffect")
        .def("GetPriority", &StatusEffect::GetPriority)
        .def("GetDescription", &StatusEffect::GetDescription)
        .def("GetName", &StatusEffect::GetName);
}

I want to use it inside python from a function declared inside a header as so:

Primaries.h

#include <boost\python.hpp>
#include <StatusEffects\StatusEffect.h>
#include "ScriptDeclarations.h";
extern void Test();

and in Primaries.cpp

#include "Primaries.h"
class StatusEffect;
using namespace boost::python;

class CppClass {
public:
  int getNum() {
    return 7;
  }
};

        BOOST_PYTHON_MODULE(CppMod) {
  class_<CppClass>("CppClass")
    .def("getNum",&CppClass::getNum);
}
void Test()
{
    CppClass mClass;
    try 
    {
    PyImport_AppendInittab("CppMod", &initCppMod);
    PyImport_AppendInittab("StatusEffect", &initStatusEffect);
    Py_Initialize();

    object main_module((
      handle<>(borrowed(PyImport_AddModule("__main__")))));

    object main_namespace = main_module.attr("__dict__");

    StatusEffect effect("Haste");
    main_namespace["eff"] = ptr(&effect);
    handle<> ignored((PyRun_String("print eff.GetName()\n",
                                    Py_file_input,
                                    main_namespace.ptr(),
                                    main_namespace.ptr() ) ));

    /*main_namespace["CppClass"] = class_<CppClass>("CppClass")
                               .def("getNum",&CppClass::getNum);
    main_namespace["cpp"] = ptr(&mClass);
    handle<> ignored(( PyRun_String("print cpp.getNum()\n",
                                     Py_file_input,
                                     main_namespace.ptr(),
                                     main_namespace.ptr() ) ));*/

  } 
  catch( error_already_set ) 
  {
    PyErr_Print();
  }
}

however, when I do this, I get the following error:

Error 16 error LNK2001: unresolved external symbol “public: class std::basic_string,class std::allocator > __thiscall StatusEffect::GetDescription(void)” (?GetDescription@StatusEffect@@QAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) C:\KHMP\Game_Cpp\KHMPCpp\KHMPCpp\GameCharacter.obj KHMPCpp

I’m quite sure this is related to my python declaration as when I remove that, and the corresponding code from Test() it compiles correctly. If I uncomment the code concerning the CppCLass in Primaries.cpp and use CppClass however, everything works fine. Do python module declarations need to be in the same source file as where they’re used? If so, is there some way around that? I’d like to ultimately be able to put all the Python wrappers for my classes in a single header file.

Can anyone guide me as to what I’m doing wrong?

thanks

Answer 1

One thing to note is that your wrapper should be in your StatusEffect.cpp, not StatusEffect.h or a separate CPP file. However, it should be OK anyway, since everything should still get exported. I would also write a test script in Python, instead of trying to test it from C.

There is not enough information to figure out why your code is not working. Some of the things to consider:

1. Does GetDescription have an implementation? Is it a pure-virtual function?
2. Is your StatusEffect class in a different module? If it is, is it __dllexport’ed (if you’re on Windows)?

The error seems to point at a problem before you get to the boost::python wrapper. Have you tried commenting out your python stuff and just trying to call StatusEffect::GetDescription directly from your test program?