[Cython] Support for C++ scoped enums (PR #1603)

[Ashwin Srinath]

Hello Cython Developers,

I just opened PR https://github.com/cython/cython/pull/3640, which
adds support for C++ scoped enumerations.

Feedback on the high-level approach used here is greatly welcome and
appreciated.  In general, I used the advice in
https://groups.google.com/forum/#!topic/cython-users/OdvjbZ1_YKU as a
starting point.

One problem I haven't been to solve is coercing Python objects to and
from the enum class correctly. Currently the following utility
function does the conversion from C++->Python (CppConvert.pyx):

  #################### enum.to_py ####################
  cdef extern from "<type_traits>" namespace "std":
      cdef cppclass underlying_type_t[T]:
          pass

  @cname("{{cname}}")
  cdef object {{cname}}(const {{TYPE}}& x):
       return <int><underlying_type_t[{{TYPE}}]>(x)

As you can see, the conversion goes
C++->underlying_type_t->int->Python, where ideally I'd like to
eliminate the conversion to `int` before obtaining a Python object.

Any ideas for how to solve this? If you prefer that this discussion
happen more broadly on GitHub, I'm happy to move it there.

Best,
Ashwin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/cython-devel/attachments/20200526/f9850fae/attachment.html>