[C++-sig] Passing Python classes derived from C++ back into C++

[Paul Scruby]

Hi Rento,

Got it working now.  I did not realize that the wrapper<> automatically 
handled downcast conversion as well.

Thanks,

Paul

"Renato Araujo" <renatox at gmail.com> wrote in message 
news:95291a80907070708p66393027nde354f5b7cc3ab1a at mail.gmail.com...
Hi Paul,

You can  extend from boost::python::wrapper<T>  and use this function
to get PyObject from c++ object.
namespace detail
{
..
  namespace wrapper_base_ // ADL disabler
  {
    inline PyObject* get_owner(wrapper_base const volatile& w);
....

BR



On Tue, Jul 7, 2009 at 7:54 AM, Paul Scruby<paul at gingernut.tv> wrote:
> Hiya,
>
> I'm trying to pass a C++ class that I've extended in Python to a C++
> function. I found an example of how to do this in David Abrahams article 
> on
> Building Hybrid Systems with Boost Python.
> http://www.boostpro.com/writing/bpl.html#virtual-functions
>
> However, I've not managed to get this example to compile:
>
> #include <string>
> #include <boost/python.hpp>
> using namespace boost::python;
>
> class Base
> {
> public:
> virtual int f(std::string x) { return 42; }
> virtual ~Base() {}
> };
>
> int calls_f(Base & b, std::string x) { return b.f(x); }
>
> struct BaseWrap : Base
> {
> BaseWrap(PyObject * self_) : self(self_) {}
> PyObject * self;
> int f_default(std::string x) { return this->Base::f(x); }
> int f(std::string x) { return call_method<int>(self, "f", x); }
> };
>
> BOOST_PYTHON_MODULE(mytest)
> {
> def("calls_f", calls_f);
> class_<Base, BaseWrap>("Base")
> .def("f", &Base::f, &BaseWrap::f_default);
> }
>
> _________________
>
> from mytest import *
>
> class Derived(Base):
> def f(self, s):
> return len(s)
>
> calls_f(Base(), 'foo')
> calls_f(Derived(), 'forty-two')
>
>
> With Boost 1.38 and Visual Studio 8 compiler I get the error:
>
> boost_1_38\boost\python\object\make_instance.hpp(68) : see reference to
> function template instantiation
> 'boost::python::objects::value_holder_back_reference<Value,Held>::value_holder_back_reference<boost::reference_wrapper<T>>(PyObject
> *,A0)' being compiled
> with
> [
> Value=Base,
> Held=BaseWrap,
> T=const Base,
> A0=boost::reference_wrapper<const Base>
> ]
>
> With Boost 1.63 and Sun C++ 5.9 compiler I get the error:
>
> boost-1_36/boost/python/object/class_metadata.hpp", line 232:
> Error: Overloading ambiguity between "static
> boost::python::objects::class_metadata<Base, BaseWrap,
> boost::python::detail::not_specified,
> boost::python::detail::not_specified>::maybe_register_pointer_to_python(void*,
> void*, void*)"
> and "static boost::python::objects::class_metadata<Base, BaseWrap,
> boost::python::detail::not_specified,
> boost::python::detail::not_specified>::maybe_register_pointer_to_python(void*,
> void*, mpl_::bool_<1>*)".
>
> I also tried extending the boost::python::wrapper<T> using get_override()
> instead of call_method<T>(), but as it can not be constructed with a
> PyObject * so the C++ base class can't be extracted from a Python
> derivation.
>
> Has anyone managed to get this working? Any help will be gratefully
> received.
>
> Cheers,
>
> Paul
>
>
>
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-- 
Renato Araujo Oliveira Filho