How to build a cython wrapper for c++ function with stl list parameter

Question

Consider the following simple c++ code. (printlist.h)

#ifndef TESTLIB_H
#define TESTLIB_H

#include <iostream>
#include <list>

void printlist(std::list<int> &);

#endif

(printlist.c)

#include "printlist.h"

using namespace std;

void printlist(list<int> &l)
  {
  for(list<int>::const_iterator i = l.begin(); i != l.end(); i++)
  cout << *i << ' ';
  cout << endl;
  }

My question is how to use this code with cython, the difficulty being the fact that printlist takes a stl::list. Is there a way to declare this using "extern"? If not, what is the simplest way to use this function.

This is my attempt:

(test.pyx)

 from libcpp.list cimport list

 cdef extern from "printlist.h":
         void printlist(std::list<int> &)

 cdef list[int] l = range(10)
 printlist(l)

(setup.py)

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

 ext_modules = [Extension("test", ["test.pyx", "printlist.C"], language='c++',)]

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

The error message I get is the following:

Error compiling Cython file:
------------------------------------------------------------
...
from libcpp.list cimport list

cdef extern from "printlist.h":
        void printlist(std::list<int> &)
                          ^
------------------------------------------------------------

test.pyx:4:27: Expected an identifier or literal

Answer 1

There are two primary issues with your code.

The first one is that you are using C++ in a C file. Start by renaming printlist.c to printlist.cpp

The second is that when you define the calling signature for the external function you use the C++ syntax for the function declaration instead of using braces for class templates as is described at http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html#templates . This is what is causing the error you are seeing. The line

void printlist(std::list<int> &)

should be replaced by

void printlist(list[int] &)

You seem to have done this correctly later in the Cython file when declaring a new list.

Another thing that is worth noting is that Cython files are not executed upon compilation. It might be easier to wrap your test case in a function that is callable from Python. Here is a working example.

printlist.h (This is the same as yours)

#ifndef TESTLIB_H
#define TESTLIB_H

#include <iostream>
#include <list>

void printlist(std::list<int> &);

#endif

printlist.cpp (I only changed the file extension and the spacing)

#include "printlist.h"

using namespace std;

void printlist(list<int> &l){
    for(list<int>::const_iterator i = l.begin(); i != l.end(); i++)
    cout << *i << ' ';
    cout << endl;}

test.pyx Notice that Cython can convert automatically from a Python list to a C++ list. I'm doing this in the typing of the arguments for the list_test function. It also supports some other types of automated conversions as is mentioned at http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html#standard-library

from libcpp.list cimport list

cdef extern from "printlist.h":
    void printlist(list[int] &)

def list_test(list[int] l):
    printlist(l)

setup.py (I changed the file extension here as well)

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

ext_modules = [Extension("test", ["test.pyx", "printlist.cpp"], language='c++',)]

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

test.py This is a Python script to call the list_test function I added to test.pyx.

from test import list_test

list_test([1, 2, 3])

When run, it should print the string "1 2 3".