Re: [boost] Re: Python and shared_ptr

From: David Abrahams (david.abrahams_at_[hidden])
Date: 2001-09-23 20:34:49

Tim,

Your python_ptr<T> idea might well work... but a simpler approach is to have
your C++ functions take arguments of boost::python::ref (which is a generic
reference-counted PyObject). Then you can call
from_python(x.get(),type<T*>()) to check that you really have a T object,
and get the T* out of it. In fact, you might implement your python_ptr<T> in
terms of this approach.

HTH,
Dave

----- Original Message -----
From: <timfisken_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Sunday, September 23, 2001 10:34 AM
Subject: [boost] Re: Python and shared_ptr

> --- In boost_at_y..., "David Abrahams" <david.abrahams_at_r...> wrote:
> > Yeah, you are missing something, though I have to admit it's a
> subtle point.
> > It took me quite a while to figure out what was going on with your
> case. The
> > problem is that because boost::shared_ptr<> uses a non-intrusive
> reference
> > count, you can't create a shared_ptr to any arbitrary object.
> Boost.Python
> > was designed to be able to handle objects returned from C++ held by
> all
> > kinds of smart pointers (e.g. std::auto_ptr). Given an arbitrary
> object of
> > type T, it isn't possible to safely create a shared_ptr<T>, because
> the T's
> > lifetime may be managed by some auto_ptr<> or even some other
> shared_ptr<>.
>
> I think I see. It did occour to me that using non-intrusive reference
> counted smart pointers with the reference counted python objects
> would be difficult, but when I saw the various to_python and
> from_python functions for shared_ptr, I assumed Boost.Python was
> doing something clever. Which it is, but unfortunately it isn't being
> quite as magic as I had hoped.
>
> Would it be possible for Boost.Python to include some kind of smart
> pointer for holding objects passed from python? Or perhaps it does
> already. What I mean is, a class which uses intrusive reference
> counting on the PyObject of the relevant extension-class. This would
> allow a C++ class to keep pointers to objects passed to it from
> python.
>
> Perhaps there is another way of accomplishing this. What I want to
> implement is effectively a plugin system - the majority of my program
> is in C++, with an abstract base plugin class. Then, you can create
> plugins by deriving from this class in python and passing an instance
> of this python class to a C++ plugin-manager, which stores them for
> later use. I can't store a copy of the object without slicing, and I
> can't store a raw pointer, because the python object will go out of
> scope.
>
> I can't see a way of doing this at the moment. If my understanding of
> how Boost.Python works is correct, though, a python_ptr<T> as I
> described it above shoudln't be too hard to produce. Something like
> this:
>
> template<class T>
> class python_ptr<T>
> {
>   T* actual_object_;
>   PyObject* python_object_;
> public:
>   python_ptr(PyObject* p)
>   {
>     actual_object_ = from_python<T*>(p);
>     python_object_ = p;
>     Py_XINCREF(p);
>   }
>
>   ~python_ptr()
>   {
>     Py_XDECREF(python_object_);
>   }
> };
>
> With the relevant operator->() etc., and to_python and from_python
> functions. Does this sound like something that would work? Or is
> there a better way to accomplish what I want?
>
> Thanks for your help, anyway.
>
> --
> Tim Fisken
> 223 King's College
> 07740 928082
>
>
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