[C++-sig] Wrappend objects identity problem

[David Abrahams]

Victor Nakoryakov <nail-mail at mail.ru> writes:

> Thanks for your reply.
>
> David Abrahams wrote:
>> 
>> It sounds like you want a smart pointer that, when converted to Python
>> the first time, manages a Python object.  That would make an
>> interesting addition to Boost.Python.
>> 
>
> Yes, I think this is what I want :)
>
>
> So... If I have a function in wrapped class 'A' that returns 
> shared_ptr<B> where 'B' is another wrapped class then to get always the 
> same py address of 'B' I have to write thin wrapper, that will convert 
> shared_ptr<B> to python::object and expose that function instead of 
> original. Am I right?

No, that won't do it.

Consider:

  python::object thin_wrapper()
  {
      boost::shared_ptr<B> p = create_B();

      // No Python object exists
      python::object x(p);

      // Now x holds a reference to a *NEW* Python object
      // holding *p via a copy of p
      return x;
  }

That's a new Python object **every time thin_wrapper is called**.

Here's something you might use:

   template <class T>
   struct associate_python_object(boost::shared_ptr<T>& p)
   {
       // If p isn't managing a Python object already, create one
       python::object x(p);

       // Replace p with a shared_ptr that manages the Python object
       p = python::extract<boost::shared_ptr<T> >(x);
   }

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com