Extend and Inherit multiple classes and return them back to cpp

[ahcorde]

Hi, I have been playing around with extend and inherit multiple classes and return them back to cpp. But I'm not able to do it. I wonder if it is possible.

class System
{
  /// \brief Constructor
  public: System() = default;

  /// \brief Destructor
  public: virtual ~System() = default;
};

class ISystemPreUpdate {
  public: virtual void PreUpdate() = 0;
};

class PyISystemPreUpdate: public ISystemPreUpdate
{
public:
  using ISystemPreUpdate::ISystemPreUpdate;
  public: void PreUpdate() override
              {
                PYBIND11_OVERLOAD_PURE(
                    void, /* Return type */
                    ISystemPreUpdate,      /* Parent class */
                    PreUpdate,          /* Name of function */
                          /* Argument(s) */
                );
              }
};

class TestCast
{
public:
  TestCast(std::shared_ptr<System> &_system)
  {
    auto system_pointer = _system.get();
    if (system_pointer)
    {
      std::cerr << "system_pointer is fine" << '\n';
    }
    else
    {
      std::cerr << "system_pointer is unavailable" << '\n';
    }
    auto system_preupdate = dynamic_cast<ISystemPreUpdate *>(_system.get());
    if (system_preupdate)
    {
      std::cerr << "system_preupdate is fine" << '\n';
    }
    else
    {
      std::cerr << "system_preupdate is unavailable" << '\n';
    }
  }
};

PYBIND11_MODULE(inherit_extension, m)
{
  py::class_<
    System, std::shared_ptr<System>>(m, "System", py::multiple_inheritance())
  .def(py::init<>());

  py::class_<
    ISystemPreUpdate, PyISystemPreUpdate /* <--- trampoline*/>
      pyISystemPreUpdate(m, "ISystemPreUpdate", py::multiple_inheritance());
  pyISystemPreUpdate
    .def(py::init_alias<>())
    .def("pre_update", &ISystemPreUpdate::PreUpdate);

  py::class_<TestCast>(m, "TestCast")
    .def(py::init<std::shared_ptr<System>&>());
}

#!/usr/bin/python3
import inherit_extension as m

class MyClass(m.System, m.ISystemPreUpdate):

    def __init__(self):
        m.System.__init__(self)
        m.ISystemPreUpdate.__init__(self)

    def pre_update(self):
        print('preupdate python')

ma = MyClass()

m.TestCast(ma)

Desired output

system_pointer is fine
system_preupdate is fine

Current output

system_pointer is fine
system_preupdate is unavailable

Thank you!

[EricCousineau-TRI]

I believe the issue you're facing is that the relation between MyClass, System, and ISystemPreUpdate is not exposed to C++ RTTI, so dynamic_cast<ISystemPreUpate*>(system.get()) will always return nullptr.

Possible solutions:

1. Rely on Python to extract the pointer to the ISystemPreUpdate portion; in this case, you'd write something like this (converting TestCast to a method for simplicity):

m.def("test_cast_method", [](py::handle h) {
  System* system = py::cast<System*>(h);
  ISystemPreUpdate* interface_system = py::cast<ISystemPreUpdate*>(h);
  ...
});

2. If it makes sense, ensure that System and ISystemPreUpdate are directly related (one is the parent of the other) in the C++ hierarchy.