[C++-sig] how to handle bare references with a shared_ptrHeldType

[Gregory Price]

David Abrahams wrote:
> > OTOH, if you use return_internal_reference, BP should know that 
> > creating a
> > shared_ptr with a refcount of 1 is evil..  is that really 
> what it does?
> 
> No, that's not what it does.  When you use 
> return_internal_reference it creates a Python object holding 
> the C++ object with a raw pointer. Isn't that in the documentation?

That's good news.

But I'm puzzled how that can be.
Perhaps it'd be clearer if you explain how
the following code can work:

----- begin test.cpp -----
#include <boost/shared_ptr.hpp>
#include <boost/python.hpp>

using namespace boost::python;

struct A {
  int x;
};

int
get_x(boost::shared_ptr<A> a)
{
  return a->x;
}

A&
make_A(int x)
{
  static A a;
  a.x = x;
  return a;
}

BOOST_PYTHON_MODULE(test)
{
  class_<A, boost::shared_ptr<A> >("A");
  def("get_x", &get_x);
  def("make_A", &make_A, return_internal_reference<>());
}
----- end test.cpp -----

  $ python
  >>> from test import make_A, get_x
  >>> a = make_A(3)
  >>> get_x(a)
  3

Here I use return_internal_reference on make_A;
so I should get a Python object "a" holding the C++ A
as a raw pointer.  But there's no trouble passing "a"
to get_x, which wants a shared_ptr<A>.
What is Boost.Python doing that makes this work?


Thanks for your help and patience.

Greg

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