[C++-sig] boost python exposing const*

[Holger Joukl]

Hi,

> I have a c++ class T that I succesfully expose via:
>
> class_<T, T*> ...
>
> later on, I have
>
> struct S {
>   const T* underlying;
> };
>
> class_<S,S*>("S").
>     .def_readonly("underlying", &S::underlying)
> ;
>
> From python, when I call:
>
> s.underlying
>
> where s is a python instance of S, i get the following error:
>
> TypeError: No to_python (by-value) converter found for C++ type: T const*
>
> Do I need another converter for T const* in particular? or for both
> T* and T const*?
> Does the class_<T,T*> above only expose T itself, not pointers to it?

Using add_property() instead should be your friend, to be able to use a
call policy.

E.g.:

#include <boost/python.hpp>

namespace bp = boost::python;


// classes to expose
struct A
{
};


struct B
{
    B() : underlying(NULL)  { }
    A const * underlying;
};


// expose to python
BOOST_PYTHON_MODULE(properties)
{

    bp::class_<A>("A")
        ;

    bp::class_<B>("B")
        // Won't work: "No to_python (by-value) converter found...
        //.def_readonly("underlying", &B::underlying)
        .add_property("underlying", bp::make_getter(&B::underlying,
bp::return_internal_reference<>()))
        ;
}


See FAQ entry also:
http://www.boost.org/doc/libs/1_59_0/libs/python/doc/v2/faq.html#topythonconversionfailed

Holger

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