Xtykutl

I wrap the template classes A and B through boost python, and try to cast between them. I implement the toB function and wrap it with return_internal_reference<> to achieve this. However, it doesn't return the B object in python, but returns the "dead" A object, of which member function can't be used anymore.

I narrow down the problem, which is the virtual function in the base class, Bar. The toB function correctly returns the B object referencing to A object if I remove the virtual keyword for hi(). But, what is the correct way to do this? Why does the virtual function in the base class affect the outcome? On the other hand, is there a way to cast boost python wrapping classes directly in Python?

#include <boost/python.hpp>

#include <iostream>

using namespace boost::python;



struct Bar

{

  virtual void hi() { std::cout << "Hi Bar." << std::endl; }

  virtual ~Bar() {}

};



template<typename T>

struct Foo : Bar

{

  void set(T v) { val = v; }

  T get() { return val; }

private:

  T val;

};



using A = Foo<unsigned short>;

using B = Foo<short>;



B & toB(A & a) { return *reinterpret_cast<B*>(&a);}



BOOST_PYTHON_MODULE(example)

{

  class_<A>("A")

    .def("set", &A::set)

    .def("get", &A::get)

  ;

  class_<B>("B")

    .def("set", &B::set)

    .def("get", &B::get)

  ;

  def("toB", &toB, return_internal_reference<>());

}

Python execution

>>> from example import *

>>> a = A()

>>> toB(a)

<example.A object at 0x7f7139cd27c0>

>>> toB(a).get()

Traceback (most recent call last):

  File "<stdin>", line 1 in <module>

Boost.Python.ArgumentError: Python argument types in 

    A.get(A)

did not match C++ signature:

    get(Foo<unsigned int> {lvalue})

It works well if I remove the virtual keywords in Bar.

struct Bar

{

  void hi() { std::cout << "Hi Bar." << std::endl; }

};

>>> from example import *

>>> a = A()

>>> toB(a)

<example.B object at 0x7f511000c7c0>

>>> toB(a).get()

>>> 4.484155085839415e-44

Exposed classes A and B don't provide all necessary information about their structure to python. Specifically that they derive from class Bar.
To fix the problem expose also class Bar to python and inform Boost.Python of the inheritance relationship between Bar and A and between Bar and B:

BOOST_PYTHON_MODULE(example)

{

  class_<Bar>("Bar")

    .def("hi", &Bar::hi)

  ;

  class_<A, bases<Bar>>("A")

    .def("set", &A::set)

    .def("get", &A::get)

  ;

  class_<B, bases<Bar>>("B")

    .def("set", &B::set)

    .def("get", &B::get)

  ;

  def("toB", &toB, return_internal_reference<>());

}

After that is done everything should work as expected:

>>> from example import *

>>> a=A()

>>> b=toB(a)

>>> b.get()

0