Calling external c++ template functions within Cython

Question

I have a number of c++ template functions declared and implemented in a c++ header file and I want to access some of the functions within Cython.

Suppose the c++ code is in header.hpp as follows

template <class T> 
T doublit(T& x) {
    return 2*x;
}

What do I need to write in the .pyx file and in the setup.py file so that I can use the function in Python as

>>> import modname
>>> print modname.doublit(3)
6

PS: Is it possible to access the same functions within PYPY? And, if yes, how?

---

Thanks for helping. But I have further difficulties (below) as I try to follow your way.

doublit.h

template <class T> 
T doublit(T& x) {
   return 2*x;
}

cdoublit.pxd

cdef extern from "doublit.h":
    cdef int doublit1 "doublit<int>"(int& foo)
    cdef double doublit2 "doublit<double>"(double& foo)

doublit.pyx

# main.pyx file
from cdoublit cimport *

cdef int n1 = 5
cdef double n2 = 5.0
print(doublit1(n1))
print(doublit2(n2))

and setup.py

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("doublit", ["doublit.pyx"])]

setup(
 name = 'Learning Cython',
 cmdclass = {'build_ext': build_ext},
 ext_modules = ext_modules
)

Finally, I build as

>>> python setup.py build_ext --inplace

but I get the following exception:

###:doublit markma$ python setup.py build_ext --inplace
running build_ext
cythoning doublit.pyx to doublit.c
building 'doublit' extension
creating build
creating build/temp.macosx-10.6-intel-2.7
gcc-4.2 -fno-strict-aliasing -fno-common -dynamic -isysroot /Developer/SDKs/MacOSX10.6.sdk -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c doublit.c -o build/temp.macosx-10.6-intel-2.7/doublit.o
In file included from doublit.c:311:
doublit.h:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘<’ token
doublit.c: In function ‘initdoublit’:
doublit.c:782: error: ‘doublit’ undeclared (first use in this function)
doublit.c:782: error: (Each undeclared identifier is reported only once
doublit.c:782: error: for each function it appears in.)
doublit.c:782: error: expected expression before ‘int’
doublit.c:793: error: expected expression before ‘double’
In file included from doublit.c:311:
doublit.h:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘<’ token
doublit.c: In function ‘initdoublit’:
doublit.c:782: error: ‘doublit’ undeclared (first use in this function)
doublit.c:782: error: (Each undeclared identifier is reported only once
doublit.c:782: error: for each function it appears in.)
doublit.c:782: error: expected expression before ‘int’
doublit.c:793: error: expected expression before ‘double’
lipo: can't figure out the architecture type of: /var/folders/ip/ip5rkteZFbWPEtzhmxRdVE+++Tc/-Tmp-//ccvaEGqZ.out
error: command 'gcc-4.2' failed with exit status 1

Answer 1

Cython supports template syntax but only for classes (as of Cython 0.19.1).

Although you can wrap template functions using the following syntax:

# doublit.pxd file
cdef extern from "doublit.h":
    cdef int doublit1 "doublit<int>"(int& foo)
    cdef double doublit2 "doublit<double>"(double& foo)

# main.pyx file
from doublit cimport *
cdef int n1 = 5
cdef double n2 = 5.0
print(doublit1(n1))
print(doublit2(n2))

You lose the automation, but at least you can make it work.

UPDATE

Cython 0.20 adds support for calling C++ template functions. Cython 0.20 beta release announced.