How to correctly wrap static polymorphism?

[matwey]

Hello,

I have a function at C++ side, which is template function

template<class F> void foo(F&& handler);

which accepts everything suitable to my Handler concept. For instance, F type must have operator() and cancel() member functions. I also have some predefined classes implementing Handler concept. Lets call them x_handler, y_handler, and z_handler. All of them are wrapped using pybind11 for Python side.

I would like to wrap foo function for Python side. I need the following two use-cases supported simultaneously:

1. Python user call the function with predefined handler class:

   h = XHandler(args)
   foo(h)

   In this case I want not to have double wrapping (C++ -> Python -> C++), yet I understand that I would still need some any_handler type erasure wrapper to kill the template type in order to expose the function via pybind11.
2. Python user calls the function with its own Python handler class:

   class Handler:
      def __call__(self):
         pass
      def cancel(self):
         pass
   foo(Handler())