Why can I compile as C but not as C++ with Cython on Mac OS X

Question

I am trying to figure out how to use C/C++ code in python using Cython. I can get the following example working as C code:

sum.h:

#ifndef MY_SUM_H_
#define MY_SUM_H_

int my_sum(int a, int b);

#endif

sum.c:

int my_sum(int a, int b){
    return a + b;
}

test.pyx:

cdef extern from "my_sum.h":
    cdef int my_sum(int a, int b)

cpdef sum_wrap(int a, int b):
    return my_sum(a, b)

setup.py:

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

ext_modules = [Extension("test", ["test.pyx", "my_sum.c"], language = "c")]

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

However, if I try to test it as C++ code it fails. I rename sum.c to sum.cpp and change language to c++ in setup.py. After that, it looks like this:

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

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

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

That should be enough right? It produces the following error:

$ python setup.py build_ext --inplace
running build_ext
cythoning test.pyx to test.cpp
/Users/jensrenders/miniconda3/lib/python3.7/site-packages/Cython/Compiler/Main.py:367: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: /Users/jensrenders/Dropbox/cython_demo/test.pyx
  tree = Parsing.p_module(s, pxd, full_module_name)
building 'test' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include/python3.7m -c test.cpp -o build/temp.macosx-10.7-x86_64-3.7/test.o
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard
      library instead [-Wstdlibcxx-not-found]
1 warning generated.
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include/python3.7m -c my_sum.cpp -o build/temp.macosx-10.7-x86_64-3.7/my_sum.o
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard
      library instead [-Wstdlibcxx-not-found]
1 warning generated.
g++ -bundle -undefined dynamic_lookup -L/Users/jensrenders/miniconda3/lib -arch x86_64 -L/Users/jensrenders/miniconda3/lib -arch x86_64 -arch x86_64 build/temp.macosx-10.7-x86_64-3.7/test.o build/temp.macosx-10.7-x86_64-3.7/my_sum.o -o /Users/jensrenders/Dropbox/cython_demo/test.cpython-37m-darwin.so
clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]
ld: library not found for -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: command 'g++' failed with exit status 1

I do get a test.cpp file as output but no shared object. What causes this, and how can it be solved?

Thanks for your help!

---

EDIT: As @MaximEgorushkin points out, it is strange that cython tries to compile the C++ files with gcc. I can force it to use g++ by adding os.environ["CC"] = "g++" to setup.py, but this does not solve the problem:

$ python setup.py build_ext --inplace
running build_ext
building 'test' extension
g++ -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include/python3.7m -c test.cpp -o build/temp.macosx-10.7-x86_64-3.7/test.o
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard
      library instead [-Wstdlibcxx-not-found]
1 warning generated.
g++ -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include -arch x86_64 -I/Users/jensrenders/miniconda3/include/python3.7m -c my_sum.cpp -o build/temp.macosx-10.7-x86_64-3.7/my_sum.o
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard
      library instead [-Wstdlibcxx-not-found]
1 warning generated.
g++ -bundle -undefined dynamic_lookup -L/Users/jensrenders/miniconda3/lib -arch x86_64 -L/Users/jensrenders/miniconda3/lib -arch x86_64 -arch x86_64 build/temp.macosx-10.7-x86_64-3.7/test.o build/temp.macosx-10.7-x86_64-3.7/my_sum.o -o /Users/jensrenders/Dropbox/cython_demo/test.cpython-37m-darwin.so
clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]
ld: library not found for -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: command 'g++' failed with exit status 1

Answer 1

Here is a similar problem: https://github.com/pandas-dev/pandas/issues/23424

As they suggest, and as indicated by the line in the output

clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]

adding extra_link_args=["-stdlib=libc++", "-mmacosx-version-min=10.9"] solves the problem. setup.py then looks like this:

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

ext_modules = [Extension("test",
                         sources=["test.pyx", "my_sum.cpp"],
                         language="c++",
                         extra_link_args=["-stdlib=libc++", "-mmacosx-version-min=10.9"])]

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