[C++-sig] weakref to shared_ptr conversion

[Andreas]

hello,

I have a problem with implicit conversion of weakref of shared_ptr to 
shared_ptr.

I have the following code:
//c++
class A
{
   public:
     static shared_ptr<A> create() {
       return shared_ptr<A>(new A());
     };
     ~A() {}

   private:
     A() {}
};

void foo(shared_ptr<A> pA)
{
}

BOOST_PYTHON_MODULE(Test)
{
   class_<A, shared_ptr<A> >("A", no_init)
     .def("create", &A::create)
     .staticmethod("create")
   ;
   boost::python::def("foo", foo);
}

//python
import weakref
a = A.create()
weak_a = weakref.ref(a)

foo(a)  #works fine
foo(weak_a)
#error:
#    foo(weak_a)
#Boost.Python.ArgumentError: Python argument types in
#    CodeSubWars_Common.foo(weakref)
#did not match C++ signature:
#    foo(class boost::shared_ptr<class A>)


How do i get such an implicit conversion from weakref to shared_ptr 
working? Is it possible to define an implicit converter to solve this?
(I'm using boost 1.33.1, MSVC7.1 and Python 2.4.2)

Any help would be appreciated,
andreas rose