Hi,
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 <iostream>
#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<1>()))
.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<1>()))
.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:*
import sys
sys.path[:0] = [r"bin\Release"]
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?")
*Python 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. In my real work, python crashes
after I create a second "factory" and try to add a "car" to the first
"factory".
The crash occurs in C++ "create_car" method whan trying to access the
"factory"'s "car" list.
Does anyone have insight as to what the problem is? Any useful input will
be greatly appreciated.
Greetings,
Christoff
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