Non-type template arguments C++/Cython

_func.pyx

# -*- coding: utf-8 -*-
# distutils: language = c++

from func cimport func as _func

def func(double inp):
    cdef double out
    _func[double, 3](&inp, &out);
    return out;

func.h

namespace myfunc {
template<typename T, int n>
void func(const T * const in, T * const out){
    *out = n*(*in);
}
}

func.pxd

cdef extern from "func.h" namespace "myfunc":
     cdef void func[T, int n](const T * const, T * const)

main.py

#!/usr/bin/env python  
# -*- coding: utf-8 -*-

from _func import func

if __name__ == '__main__':
    print(func(3))

setup.py

from distutils.core import setup
from Cython.Build import cythonize

ext_mod = cythonize("_func.pyx")

setup(
    ext_modules = ext_mod
)