[C++-sig] sending a c++ class to a python function

[Josh Stratton]

That worked.  Thanks.  I had trouble getting it to work as a template,
so I just removed that portion.

//template <typename T*>
inline Scene* get_pointer(QSharedPointer<Scene> const &p) {
   return p.data(); // or whatever
}


On Tue, Aug 30, 2011 at 10:57 AM, Jim Bosch <talljimbo at gmail.com> wrote:
> On 08/30/2011 07:45 AM, Josh Stratton wrote:
>>
>> Oh, okay.  So I can create a module...
>>
>> #include "scene.h"
>> BOOST_PYTHON_MODULE(scene)
>> {
>>     class_<Scene>("Scene");
>> }
>>
>> and then import it (even though) in my python I normally don't import
>> things I'm not creating.  I'm assuming this is a boost-python thing to
>> get the class into scope, which gets rid of the "converter found for
>> C++ type: Scene" error.
>
> I don't really understand what you mean; if you want to use code that was
> defined in another Python module, you always have to import it. It's just
> that in this case the module happens to be written in C++.
>
>> from scene import Scene # in my python code
>>
>> In my terminal I get...
>>
>> Error in Python:<class 'Boost.Python.ArgumentError'>: Python argument
>> types in
>>     Mesh.buildByIndex(Scene, PrimitiveParts)
>> did not match C++ signature:
>>     buildByIndex(QSharedPointer<Scene>, PrimitiveParts):   File
>> "<string>", line 21, in processFile
>>
>> for this function:
>>
>>     static void                  buildByIndex(SceneP scene,
>> PrimitiveParts parts);
>>
>> The function I'm calling has SceneP typedeffed as a
>> QSharedPointer<Scene>  and I'm assuming this error is because I haven't
>> made a Scene to QSharedPointer<Scene>  converter, which should just
>> wrap the Scene object when it comes in requiring a custom conversion
>> function.
>
> This is another case where you probably want to use something other than a
> custom converter (and if you did use a custom converter, you'd want a
> from-python lvalue converter, not a to-python converter, anyways, and those
> are defined differently).
>
> What you probably want to do is tell Boost.Python that QSharedPointer is a
> smart pointer, and tell it to wrap your Scene objects inside one (at least
> if you're going to have to deal with them a lot).
>
> To do that, you'll want to specialize boost::python::pointee and provide a
> get_pointer function:
>
> namespace boost { namespace python {
>
> template <typename T>
> struct pointee< QSharedPointer<T> > {
>    typedef T type;
> };
>
> }}
>
> // in some namespace where ADL will find it...
> template <typename T*>
> inline Scene * get_pointer(QSharedPointer<T> const & p) {
>    return p.get(); // or whatever
> }
>
> Then, when you define the class, use:
>
> class_< Scene, QSharedPointer<Scene> >(...)
>
> You can find more information in the reference documentation:
>
> http://www.boost.org/doc/libs/1_47_0/libs/python/doc/v2/class.html#classes
>
> Good Luck!
>
> Jim
>