[C++-sig] How can I pass derived Python instances to C++?

[James Amundson]

On 09/08/2011 12:17 PM, Stefan Seefeld wrote:
> On 2011-09-08 13:01, James Amundson wrote:
>> I would like to be able to create a Python class derived from a C++
>> class, then pass a instance of the Python class back to C++. It isn't
>> clear to me if there is an easy way to do this.
>>
>>
>> import foo
>> class Bar(foo.Foo):
>>      def __init__(self):
>>          pass
> You need to call the base class constructor explicitly in Python. That
> should do the trick.
Yes. Of course. Thanks! That should have been obvious to me.

In case anyone else is reading, I'll answer my own next question and say 
that Stefan's advice is necessary but not sufficient. In order to get my 
example to work, I had to read up on overridable virtual functions in 
Boost Python, e.g. here:
     http://wiki.python.org/moin/boost.python/OverridableVirtualFunctions

Here is my working example:

C++:

#include <boost/python.hpp>
#include <iostream>
class Foo
{
public:
     Foo()
     {
     }
     ;
     virtual void
     doit()
     {
         std::cout << "doing it Foo style\n";
     }
     ;
};
void
foodoer(Foo & foo)
{
     foo.doit();
}

using namespace boost::python;

struct Foo_callback : Foo
{
     Foo_callback(PyObject *p) :
         Foo(), self(p)
     {
     }
     Foo_callback(PyObject *p, const Foo& foo) :
         Foo(foo), self(p)
     {
     }
     void
     doit()
     {
         return call_method<void > (self, "doit");
     }
     static void
     default_doit(Foo& self_)
     {
         self_.Foo::doit();
     }
private:
     PyObject* self;
};

BOOST_PYTHON_MODULE(foo)
{
     class_<Foo, Foo_callback >("Foo", init<>())
     .def("doit", &Foo_callback::default_doit);
     def("foodoer", foodoer);
}

Python:

import foo
class Bar(foo.Foo):
     def __init__(self):
         foo.Foo.__init__(self)
     def doit(self):
         print "doing it Bar style"

f = foo.Foo()
foo.foodoer(f)

b = Bar()
foo.foodoer(b)


--Jim