C++/Boost Python class overriding

Question

Ok. My question is actually quite simple. This is a frame listener class and python wrapper class that is then used by boost python:

class FrameListener
    {
    public:

        struct FrameEvent
        {
            double TimeSinceLastEvent;
            double TimeSinceLastFrame;
        };

        FrameListener() {}
        ~FrameListener() {}

        virtual void FrameStart(FrameEvent& evt) = 0;
        virtual void FrameEnd(FrameEvent& evt) = 0;
        virtual void OnFrame(FrameEvent& evt) = 0;
    };

    class FrameListener_PyWrapper : public FrameListener, public boost::python::wrapper<FrameListener>
    {
    public:
        FrameListener_PyWrapper() {}    
        ~FrameListener_PyWrapper();

        void FrameStart(FrameEvent& evt);
        void FrameEnd(FrameEvent& evt);
        void OnFrame(FrameEvent& evt);
    };

Classes are then wrapped to boost python:

class_<SL_Engine::FrameListener::FrameEvent>("FrameEvent")
        .def_readonly("TimeSinceLastEvent", &SL_Engine::FrameListener::FrameEvent::TimeSinceLastEvent)
        .def_readonly("TimeSinceLastFrame", &SL_Engine::FrameListener::FrameEvent::TimeSinceLastFrame);

    class_<SL_Engine::FrameListener_PyWrapper, boost::noncopyable>("FrameListener")
        .def("OnFrame", pure_virtual(&SL_Engine::FrameListener::OnFrame))
        .def("FrameStart", pure_virtual(&SL_Engine::FrameListener::FrameStart))
        .def("FrameEnd", pure_virtual(&SL_Engine::FrameListener::FrameEnd))
        ;

This can be overriden in python to create own frame listener class:

class SimpleListen(FrameListener):
def __init__(self):
    pass
def FrameEndd(self, evt):
    pass

Then you create instance of the new SimpleListener class and "register" it to the engine with CORE_ENGINE.AddFrameListener() function. The function then uses boost::python::extract() to extract it and put it on std::vector of frame listeners which are then executed every frame. This works perfectly, but if I forget to define base in simple class or misspell it, it obviously results in a crash. I have searched on google both in boost::python or C/Python API, how can I test if base class is properly defined. The AddFrameListener takes PyObject* argument. I want to test if PyObject is class type first. If you accidently try to put an integer value or a string that the program will identify the error and message that the object is not class type. If it is class type I want to test if it has base class and if it can be extracted to FrameListener_PyWrapper*. Later on I want to test each individual function (FrameStart, FrameEnd and OnFrame) if they are overriden in python class and if arguments match. How can I get list of arguments from python function? If any of the functions from python class is not overriden it will give me a warning. If arguments don't match and you try to call:

if (boost::python::override f = this->get_override("FrameEnd"))
    {
        f(evt);
        return;
    }

It will result in a crash without any information about error. I want to check if arguments match when calling AddFrameListener function and save information if any of the functions are not properly overriden the engine will not call them.

UltimateDRT · Accepted Answer · 2017-08-24 12:25:59Z

The crash maybe related to the fact you are calling into Python without the GIL locked. You should lock it everytime you interact with a Python Object:

    /*
This RAII structure ensures that threads created on the native C side
adhere to the laws of Python and ensure they grab the GIL lock when
calling into python
*/
struct PyLockGIL
{

    PyLockGIL()
        : gstate(PyGILState_Ensure())
    { 
    }

    ~PyLockGIL()
    {
        PyGILState_Release(gstate);
    }

    PyLockGIL(const PyLockGIL&) = delete;
    PyLockGIL& operator=(const PyLockGIL&) = delete;

    PyGILState_STATE gstate;
};

Then use it before you callback into Python:

    if (override fe = this->get_override("FrameEnd"))
{
    PyLockGIL lock;
    try
    {
        fe();
        return;
    }
    catch (const error_already_set&)
    {
        std::cout << "Exception in FrameEnd in application";
        PyErr_Print();
    }
}
else
{
    std::cout << "FrameEnd called but not overridden";
}

Collectives™ on Stack Overflow

C++/Boost Python class overriding

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related