Re: [boost] boost::python: to-python conversion on polymorphic class

Subject: Re: [boost] boost::python: to-python conversion on polymorphic class
From: Allan Johns (allan.johns_at_[hidden])
Date: 2010-12-15 17:20:59

On Thu, Dec 16, 2010 at 7:18 AM, Dave Abrahams <dave_at_[hidden]> wrote:

> At Mon, 13 Dec 2010 10:56:25 +1100,
> Allan Johns wrote:
> >
> > Right you are.
> >
> > With regards to my original snippet:
> >
> >
> >     template<typename Attrib>
> >     struct typed_attrib_topython
> >     {
> >         static PyObject* convert(const Attrib& a)
> >         {
> >             return bp::incref(bp::object(a.value()).ptr());  // return
> POD
> > type
> >         }
> >     };
> >
> >     template<typename T>
> >     struct TypedAttribBind
> >     {
> >         typedef TypedAttribute<T> attrib_t;
> >
> >         TypedAttribBind()
> >         {
> >             boost::python::to_python_converter<attrib_t,
> > typed_attrib_topython<attrib_t> >();
> >         }
> >     };
> >
> >
> > TypedAttribute<T> derives from Attribute, which derives from Object (the
> > common polymorphic base class).
> > TypedAttribute<T>.value() is a function which returns the POD type
> > associated with the attribute.
> > TypedAttribBind<T> is instantiated for a range of POD types T, in order
> to
> > register the to-python converters.
>
> > This doesn't behave as expected - if I call any bound function which
> > returns a shared_ptr<Object> from python (where the object is
> > actually a TypedAttribute<T>), I just get a wrapped Object back,
> > rather than a POD type.
>
> I *think* what you mean is that you don't get a Python built-in type
> (since POD is a C++ concept and you're looking at the resulting object
> from the Python side)?
>

Yes sorry I was inferring built-in python types that are analogous to C++
POD types (float, int etc).

>
> I'm afraid shared_ptr<T> is treated specially by Boost.Python at an
> extremely low level, so this is hard to do.  However, although I can't
> support this usage, you might take a look in
> boost/python/converter/shared_ptr_to_python.hpp and consider creating
> an overload (in namespace boost::python::converter) for
> shared_ptr<Object>.
>

Ok thanks, I'll take a look at that.

Cheers!
A

>
> --
> Dave Abrahams
> BoostPro Computing
> http://www.boostpro.com
>
> _______________________________________________
> Unsubscribe & other changes:
> http://lists.boost.org/mailman/listinfo.cgi/boost
>