[Boost-users] [Boost.Python] newbie: Lifetime of objects

20 Dec 2005

      Hello everyone.

I'm trying to map into python classes shown below.

class A 
{
    friend class B;

public:
    ~A(void) {}

private:
    A(B& b)
    : m_B(&b)
    {
    }

    B* m_B;
};

class B
{
    friend class C;

public:
    ~B(void) {}

    A* make_a(void)
    {
        return new A(*this);
    }

private:
    B(C& c)
    : m_C(&c)
    {
    }

    C* m_C;
};

class C
{
public:
    ~C(void) {}

    B* make_b(void)
    {
        return new B(*this);
    }
};

C* make_c()
{
    return new C();
}

Objects are supposed to be created in the order C, B, A and destroyed 
in the
order A, B, C.
I do not want to map constructors of A, B and C.
I can see three ways to do what I want:

1) 
    class_<A>("A", no_init);
    class_<B>("B", no_init)
    .def("make_a", &B::make_a, return_internal_reference<>())
    ;
    class_<C>("C", no_init)
    .def("make_b", &C::make_b, return_internal_reference<>())
    ;

    def("make_c", make_c, return_value_policy<manage_new_object>());

2)
    class_<A>("A", no_init);
    class_<B>("B", no_init)
    .def("make_a", &B::make_a, 
return_value_policy<manage_new_object>())
    ;
    class_<C>("C", no_init)
    .def("make_b", &C::make_b, 
return_value_policy<manage_new_object>())
    ;

    def("make_c", make_c, return_value_policy<manage_new_object>());

3)
    class_<A, shared_ptr<A>, noncopyable>("A", no_init);
    class_<B, shared_ptr<B>, noncopyable>("B", no_init)
    .def("make_a", &B::make_a, 
return_value_policy<manage_new_object>())
    ;
    class_<C, shared_ptr<C>, noncopyable>("C", no_init)
    .def("make_b", &C::make_b, 
return_value_policy<manage_new_object>())
    ;

    def("make_c", make_c, return_value_policy<manage_new_object>());

    C::make_b and B::make_a are supposed to return shared_ptr<B> and
shared_ptr<A> respectively in this case.

After debugging this code I've got strange results:
        - In case 1 objects aren't get deleted at all;
        - In case 2 objects are get deleted in wrong order;
        - In case 3 objects are get deleted in wrong order;

Cases 2 and three seem to be identical to me. The only difference I can 
see is
that in case 3 I should manage objects lifetime by myself from C++ 
code. In
opposite, in case 2 objects lifetime will be managed by python.

I couldn't get an answer to my problem neither from documentation nor 
from
Boost.Python tests nor from Google.

Any explanations or suggestions will be greatly appreciated.

Thank you.
--
Sergey Sikorskiy