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

Sergey Sikorskiy writes:

Hello everyone.

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

Sergey, The C++-sig (http://www.boost.org/more/mailing_lists.htm#cplussig) is usually a better place to get your Boost.Python questions answered.

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<>()) ;

Bad idea. That would only be appropriate if the resulting A object were internal to (and its lifetime managed by) the B object. It is not.

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

def("make_c", make_c, return_value_policy());

Better. However, it doesn't account for the fact that the C object must exist as long as the B object does. You need to compose your return_value_policy with with_custodian_and_ward<0,1>.

2)

All three of these have the same problem.

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

THe part below is the same as what you proposed in #1.

class_<C>("C", no_init) .def("make_b", &C::make_b, return_value_policy()) ;

def("make_c", make_c, return_value_policy());

3) class_("A", no_init); class_("B", no_init) .def("make_a", &B::make_a, return_value_policy()) ; class_("C", no_init) .def("make_b", &C::make_b, return_value_policy()) ;

I don't understand what you're trying to accomplish with noncopyable.

def("make_c", make_c, return_value_policy());

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

That gets you off the hook for return_value_policy, but unless you're storing shared_ptrs in A, B, and C (rather than raw pointers), you still need to use with_custodian_and_ward.

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.

Have you read http://www.boost.org/libs/python/doc/tutorial/doc/html/python/functions.html... -- Dave Abrahams Boost Consulting www.boost-consulting.com