[Boost-bugs] [Boost C++ Libraries] #11369: Boost.Python: return_internal_reference<> bug

Subject: [Boost-bugs] [Boost C++ Libraries] #11369: Boost.Python: return_internal_reference<> bug
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-06-02 06:13:07

#11369: Boost.Python: return_internal_reference<> bug
-------------------------------------------------+-------------------------
 Reporter:  Christoff Heinrich Kok               |      Owner:  rwgk
  <christoff.kok@…>                              |     Status:  new
     Type:  Bugs                                 |  Component:  Python
Milestone:  To Be Determined                     |   Severity:  Showstopper
  Version:  Boost 1.58.0                         |
 Keywords:  Python reference_internal_object     |
-------------------------------------------------+-------------------------
 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

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11369>
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