I was following the Cython tutorial on using C libraries and found out I can do the same for C++, and consulted the equivalent guide. Instead of a class, I wanted to start simple and import and use a function.

After not being succesful with the class-like syntax for defining external functions, like cdef extern from "<filename>":, I found out a way to properly compile and use the C++ code, but it’s not behaving as expected.

There are 4 files which interact with each other:

hello.cpp

#include <iostream>

void hello_from_cpp() {
    std::cout << "Hello from C++!" << std::endl;
}

cpp_hello.pyx

# distutils: language = c++
# cython: language_level = 3
cpdef extern void hello_from_cpp()

def myfun():
    hello_from_cpp()

setup.py

from setuptools import setup, Extension
from Cython.Build import cythonize

setup(
    ext_modules=cythonize([
        Extension("cpp_hello", ["cpp_hello.pyx", "hello.cpp"])
        ]),
)

main.py

import cpp_hello

if __name__ == "__main__":
    cpp_hello.hello_from_cpp()

When this is compiled (cythonized) with python setup.py build_ext --inplace or python setup.py build_ext -i, I can run python main.py and get the return Hello from C++! as expected.

The weird behavior is that removing the unused myfun() definition from cpp_hello.pyx breaks the compilation process

[1/1] Cythonizing cpp_hello.pyx
running build_ext
building 'cpp_hello' extension
creating build
creating build/temp.linux-x86_64-cpython-310
x86_64-linux-gnu-gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/usr/include/python3.10 -c cpp_hello.cpp -o build/temp.linux-x86_64-cpython-310/cpp_hello.o
cpp_hello.cpp: In function ‘int __pyx_pymod_exec_cpp_hello(PyObject*)’:
cpp_hello.cpp:3027:84: error: ‘hello_from_cpp’ was not declared in this scope
 3027 |     PyObject* wrapped = __Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(hello_from_cpp);
      |                                                                                    ^~~~~~~~~~~~~~
cpp_hello.cpp: At global scope:
cpp_hello.cpp:2362:18: warning: ‘PyObject* __Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(void (*)())’ defined but not used [-Wunused-function]
 2362 | static PyObject *__Pyx_CFunc_9cpp_hello_void__lParen__rParennoexcept_to_py_(void (*__pyx_v_f)(void)) {
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1

My end goal is to be able to create libraries of C++ code which can be used in Python programs. The .pyx and setup.py files glue them together by providing Python definitions of the C++ equivalents with cpdef instead of cdef.