[C++-sig] Wrapping virtual method returning void

[Botos, Christopher]

I have an example C++ base class (Song) with a virtual method (chorus) returning void that is called by another method (play) of the same class.  I want to be able to derive a python class from this and override the virtual method.  I experience the following behavior:

1. If I simply instantiate the wrapped base class in python and call play, **the virtual chorus is never entered/executed**.
2. If I create a derived class and override the virtual chorus, the derived chorus is executed correctly.
3. If I change the base class chorus to return int (or anything) instead of void, the base class chorus is executed correctly.

I have wrapped my class as described here: http://www.boost.org/doc/libs/1_43_0/libs/python/doc/v2/wrapper.html

My example C++,  python code, and output python output is below (I've tried to make it as concise as possible).  I can work around this issue in my application by having my virtual method return an unused object, although this is not ideal.  I would appreciate any other suggestions, whether I'm doing something incorrectly, or is this possibly a boost python bug?

//------------------------------------------------
//  Has virtual method returning void
//  Numbers indicate order in which the print statements should appear.
//------------------------------------------------
class Song
{
public:
    virtual ~Song(){}
    void play()
    {
        std::cout << "\n1) Base class --> Song.play" << std::endl;
        chorus();
        std::cout << "3) Base class <-- Song.play" << std::endl;
    }

    // THIS METHOD IS NEVER EXECUTED
    virtual void chorus()
    {
        std::cout << "2) Base class Song.chorus returning void" << std::endl;
    }
};

struct SongWrapper :  Song, bp::wrapper<Song>
{
    void chorus()
    {
        if (bp::override chorus = this->get_override("chorus"))
                chorus();
                return;
        Song::chorus();
        return;
    }

    void default_chorus()    {  this->Song::chorus(); }
};

//------------------------------------------------
//  Has virtual method returning int
//  Numbers indicate order in which the print statements should appear.
//------------------------------------------------
class AnotherSong
{
public:
    virtual ~AnotherSong(){}
    void play()
    {
        std::cout << "\n1) Base class --> AnotherSong.play" << std::endl;
        chorus();
        std::cout << "3) Base class <--AnotherSong.play" << std::endl;
    }

   // THIS METHOD (returning int) IS EXECUTED
    virtual int chorus()
    {
        std::cout << "2) Base class AnotherSong.chorus returning int" << std::endl;
        return 0;
    }
};

struct AnotherSongWrapper :  AnotherSong, bp::wrapper<AnotherSong>
{
    int chorus()
    {
        if (bp::override chorus = this->get_override("chorus"))
                return chorus();
        return AnotherSong::chorus();
    }

    int default_chorus()   {  return this->AnotherSong::chorus(); }
};

BOOST_PYTHON_MODULE(Songs)
{
    bp::class_<SongWrapper,boost::noncopyable>("Song")
       .def("play", &Song::play)
       .def("chorus", &Song::chorus, &SongWrapper::default_chorus)
       ;

    bp::class_<AnotherSongWrapper,boost::noncopyable>("AnotherSong")
       .def("play", &AnotherSong::play)
       .def("chorus", &AnotherSong::chorus, &AnotherSongWrapper::default_chorus)
       ;
}
#------------------------------------------------
# Python file ...
#------------------------------------------------
from Songs import Song, AnotherSong

# Base class chorus returning void is not executed: step 2 is missing in the output
s1 = Song()
s1.play()

# Derived class chorus is executed: step 2 in the output
class Chart(Song):
    def chorus(self):
        print "** 2) Derived class Chart.chorus"
c1 = Chart()
c1.play()

# Base class chorus returning int is executed: step 2 in output
s2 = AnotherSong()
s2.play()

#------------------------------------------------
# Python output ...
# In first case, step 2 is missing ... chorus was not executed
#------------------------------------------------
1) Base class --> Song.play
3) Base class <-- Song.play

1) Base class --> Song.play
** 2) Derived class Chart.chorus
3) Base class <-- Song.play

1) Base class --> AnotherSong.play
2) Base class AnotherSong.chorus returning int
3) Base class <--AnotherSong.play


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