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

Subject: Re: [boost] boost::python: to-python conversion on polymorphic class
From: Dave Abrahams (dave_at_[hidden])
Date: 2010-12-15 15:18:38

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)?

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>.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com