C vs. C++ in Cython.

a.pyx

# coding: utf8
# distutils: language = c++
# distutils: sources = b.cpp

# Using C calling convention works with "cdef extern ..." directly; no
# hpp header.
# Without it we need to use the cdef extern from "b.hpp": ... declaration.

# Implicitly forces C calling convenction?
cdef extern void c()

# This works with C++ just fine.
cdef extern from "b.hpp":
    void cpp_good()

# This doesn't. Undefined symbol 'cpp_bad'.
cdef extern void cpp_bad()

c()
cpp_good()
cpp_bad()

b.cpp

// Indicate C calling convention for our C function.
extern "C" { void c(); }

void c() { }
void cpp_good() { }
void cpp_bad() { }

b.hpp

void cpp_good();

// If we include cpp_bad() here, we get an error about redefining it with C
// linkage after having already defined it with C++ linkage.

setup.py

#!/usr/bin/env python
# coding: utf8

from setuptools import setup
from Cython.Build import cythonize

cython_modules = cythonize('*.pyx')
setup(ext_modules = cython_modules)