id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
11369	Boost.Python: return_internal_reference<> bug	Christoff Heinrich Kok <christoff.kok@…>	Ralf W. Grosse-Kunstleve	"I am having an issue with Boost.Python with a very simple use case.

I am returning a reference to an object, and it seems that my python object looses its C++ object's reference at a stage for some reason.

Please see my example below reproducing this issue.

C++ Code:

{{{
#include <vector>
#include <string>

#include <cmath>
#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>

class Car {
public:
    Car(std::string name) : m_name(name) {}

    bool operator==(const Car &other) const {
        return m_name == other.m_name;
    }

    std::string GetName() { return m_name; }
private:
    std::string m_name;
};

class Factory {
public:
    Factory(std::string name) : m_name(name) {}

    bool operator==(const Factory &other) const {
        return m_name == other.m_name
            && m_car_list == other.m_car_list;
    }

    Car& create_car(std::string name)
    {
        m_car_list.emplace_back(Car(name));
        return m_car_list.back();
    }

    std::string GetName() { return m_name; }
    std::vector<Car>& GetCarList() { return m_car_list;}
private:
    std::string m_name;
    std::vector<Car> m_car_list;
};

class Manufacturer {
public:
    Manufacturer(std::string name) : m_name(name) {}

    bool operator==(const Manufacturer &other) const {
        return m_name == other.m_name
            && m_factory_list == other.m_factory_list;
    }

    Factory& create_factory(std::string name)
    {
        m_factory_list.emplace_back(Factory(name));
        return m_factory_list.back();
    }

    std::string GetName() { return m_name; }
    std::vector<Factory>& GetFactoryList() { return m_factory_list;}
private:
    std::string m_name;
    std::vector<Factory> m_factory_list;
};

BOOST_PYTHON_MODULE(carManufacturer)
{
    using namespace boost::python;
    class_<Manufacturer>(""Manufacturer"", init<std::string>())
        .add_property(""factory_list"", make_function(&Manufacturer::GetFactoryList, return_internal_reference<>()))
        .add_property(""name"", &Manufacturer::GetName)
        .def(""create_factory"", &Manufacturer::create_factory, return_internal_reference<>());
    class_<Factory>(""Factory"", init<std::string>())
        .add_property(""car_list"", make_function(&Factory::GetCarList, return_internal_reference<>()))
        .add_property(""name"", &Factory::GetName)
        .def(""create_car"", &Factory::create_car, return_internal_reference<>());
    class_<Car>(""Car"", init<std::string>())
        .add_property(""name"", &Car::GetName);

    class_<std::vector<Factory> >(""FactoryList"")
        .def(vector_indexing_suite<std::vector<Factory> >());
    class_<std::vector<Car> >(""Car"")
        .def(vector_indexing_suite<std::vector<Car> >());
}
}}}

Python Code:

{{{
from carManufacturer import *

vw = Manufacturer(""VW"")
vw_bra_factory = vw.create_factory(""Brazil Factory"")
beetle = vw_bra_factory.create_car(""Beetle69"")

if vw_bra_factory is vw.factory_list[0]:
    print(""equal."")
else:
    print(""NOT EQUAL"")
print(""## I expected them to be the same reference..?"")


print(""vw_bra_factory Car List size : "" + str(len(vw_bra_factory.car_list)))
print(""Actual Car List size         : "" + str(len(vw.factory_list[0].car_list)))
print(""## This still works. Maybe the python objects differ, but refer to the same C++ object. I can live with that."")

vw_sa_factory = vw.create_factory(""South Africa Factory"")
print(""vw_bra_factory Car List size : "" + str(len(vw_bra_factory.car_list)))
print(""Actual Car List size         : "" + str(len(vw.factory_list[0].car_list)))
print(""## .. what? why? brazil py object has no cars now? I don't get it. I can't have any of that."")

print(""## What will happen if I create another car in the brazil factory?"")
combi = vw_bra_factory.create_car(""Hippie van"")
print(""vw_bra_factory Car List size : "" + str(len(vw_bra_factory.car_list)))
print(""Actual Car List size         : "" + str(len(vw.factory_list[0].car_list)))

print(""## And another."")
citi_golf = vw_bra_factory.create_car(""Citi golf"")
print(""vw_bra_factory Car List size : "" + str(len(vw_bra_factory.car_list)))
print(""Actual Car List size         : "" + str(len(vw.factory_list[0].car_list)))
print(""## 'vw_bra_factory' must have lost its C++ reference it had to 'vw.factory_list[0]' when I created a new factory. Why?"")
}}}

Output:

{{{
NOT EQUAL
## I expected them to be the same reference..?
vw_bra_factory Car List size : 1
Actual Car List size         : 1
## This still works. Maybe the python objects differ, but refer to the same C++ object. I can live with that.
vw_bra_factory Car List size : 0
Actual Car List size         : 1
## .. what? why? brazil py object has no cars now? I don't get it. I can't have any of that.
## What will happen if I create another car in the brazil factory?
vw_bra_factory Car List size : 1
Actual Car List size         : 1
## And another.
vw_bra_factory Car List size : 2
Actual Car List size         : 1
## 'vw_bra_factory' must have lost its C++ reference it had to 'vw.factory_list[0]' when I created a new factory. Why?
}}}

This is just an example made to reproduce my real work's problem in a presentable way.

Is there any workaround to this? I could not find any in bug reports, the mailing list or anywhere else.

Regards,
Christoff"	Bugs	new	To Be Determined	python USE GITHUB	Boost 1.58.0	Showstopper		Python reference_internal_object