cython: test converting py2/3 string objects to C++ std::string

libtest.cc

#include "libtest.hh"

std::string copy_string(std::string &s) {
    std::string s2 = s;
    return s2;
}

libtest.hh

#ifndef LIBCYTEST_H
#define LIBCYTEST_H

#include <string>

std::string copy_string(std::string &s);

#endif // LIBCYTEST_H

libtest.pxd

from libcpp.string cimport string

cdef extern from "libtest.hh":
    string copy_string(string &s)

test.pyx

# cython: c_string_type=unicode, c_string_encoding=utf8

cimport libtest

def test(s):
    return libtest.copy_string(s)

"""
python -c "import test; print (test.test('foo'))"

Python2:
foo

Python3:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "test.pyx", line 6, in test.test
  File "stringsource", line 15, in string.from_py.__pyx_convert_string_from_py_std__in_string
TypeError: expected bytes, str found
"""