Wrapping a C++ class in Python using SWIG

Question

example.h:

#ifndef EXAMPLE_H
#define EXAMPLE_H

class Math {
 public:
    int pi() const;
    void pi(int pi);
 private:
    int _pi;
};

#endif

example.cpp:

#include "example.h"

int Math::pi() const {
    return this->_pi;
}  
void Math::pi(int pi) {
    this->_pi = pi;
}

example.swig:

%module example
%{ 
    #define SWIG_FILE_WITH_INIT
    #include "example.h"
%}
%include "example.h"

I then generate the wrappers, "example.py" and "example_wrap.c" using:

swig  -python example.swig

When I try to compile the wrapper class using:

g++ -fPIC -c example.cpp example_wrap.c -I/usr/local/include/python2.6/

I get the following error:

example_wrap.cpp: In function "PyObject* Swig_var_Math_get()":
example_wrap.cpp:2725: error: expected primary-expression before "void"
example_wrap.cpp:2725: error: expected ")" before "void"

The Error is at the following line :

pyobj = SWIG_NewPointerObj(SWIG_as_voidptr(&Math), SWIGTYPE_p_class,  0 );

#define SWIG_as_voidptr(a) (void *)((const void *)(a))

Is it the right way to generate the wrapper class "example_wrap.c"?

Solution 1

I think the swig command should be "swig -c++ -python example.swig"

Solution 2

There's not enough information here to be sure what's wrong, but I have two ideas for things you can try.

1. Your g++ invocation is compiling a C source file as if it were C++. This is not guaranteed to work. Try instead

   gcc -I/usr/local/include/python2.6 -fPIC -c example_wrap.c
   gcc -I/usr/local/include/python2.6 -fPIC -c example.cpp
   g++ -shared example_wrap.o example.o -o example.so
   

   (yes, srsly, only use g++ for the link)

2. If that doesn't work, compile example_wrap.c like this:

   gcc -I/usr/local/include/python2.6 -fPIC -c -save-temps example_wrap.c
   

That will fail the same way but will produce a file named example_wrap.i which is the result of preprocesing. It will be gigantic. Search that file for the function Swig_var_Math_get, and add to your question the complete text of that function (but nothing else).

Comments

[jweyrich]

Oops, example.cpp should be compiled with g++. Why gcc? Avoid C++ mangling?

[zwol]

It is only necessary to use g++ to link. gcc -c does the Right Thing when applied to a .cpp source file. But g++ -c does not do the Right Thing when applied to a .c source file. So the easiest rule to remember is, always use gcc to compile, even if the source is C++; only use g++ to link a program that contains C++. Same same for any of the other languages supported by GCC: use the language-specific driver only to link. Personally I don't think the language-specific drivers should even exist, but they do let you not have to know exactly how to call for the runtime libraries.

[jweyrich]

well, yes, you can use gcc to compile C++. You could also use gcc to link C++, but you'd have to manually link against libstdc++, etc. Put that way, I find easier to use gcc for C, and g++ for C++. But it's just personal taste. The important is that problem & solution were correctly pointed out. +1