swig compiles python c++ wrapper ,gcc build error: "const char* cannot convert to char*"

Question

I was trying a very simple code piece that shows how swig converts a c++ module into a python callable, it's from internet. I followed the steps, but seems when gcc tries to compile generated code, it prompts error.

Here're my steps:

$cat example.h

#pragma once
int fact(int n);

cat example.cpp

#include "example.h"
int fact(int n) {
    if (n < 0){ /* This should probably return an error, but this is simpler */
        return 0;
    }
    if (n == 0) {
        return 1;
    }
    else {
        /* testing for overflow would be a good idea here */
        return n * fact(n-1);
    }
}

$cat example.i

%module example
%{
#define SWIG_FILE_WITH_INIT
#include "example.h"
%}
int fact(int n);

And a python DistUtil file $cat setup.cpp.py

"""
setup.py
"""

from distutils.core import setup, Extension
example_module = Extension('_example',
                           sources=['example_wrap.cxx', 'example.cpp'],
                           )
setup (name = 'example',
       version = '0.1',
       author      = "SWIG Docs",
       description = """Simple swig example from docs""",
       ext_modules = [example_module],
       py_modules = ["example"],
       )

Then use swig to compile

$ swig -c++ -python example.i

$ python setup.cpp.py build_ext --inplace

running build_ext
building '_example' extension
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/python2.7 -c example_wrap.cxx -o build/temp.linux-x86_64-2.7/example_wrap.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
In file included from example_wrap.cxx:2554:
example.h:5:20: warning: no newline at end of file
example_wrap.cxx: In function ‘int SWIG_Python_ConvertFunctionPtr(PyObject*, void**, swig_type_info*)’:
example_wrap.cxx:2051: error: invalid conversion from ‘const char*’ to ‘char*’
example_wrap.cxx: In function ‘void SWIG_Python_FixMethods(PyMethodDef*, swig_const_info*, swig_type_info**, swig_type_info**)’:
example_wrap.cxx:3200: error: invalid conversion from ‘const char*’ to ‘char*’
error: command 'gcc' failed with exit status 1

Well as you can see I'm compiling a generated file, but gcc prompts error.

I would suppose that swig shouldn't have error to deal with such an easy case, just wonder if my code, or command line option is missing something?

Need you expertise!