[C++-sig] In a C++ extension, how to use a C++ class exported in another extension

[Clark]

We know that cross-module type info share is easy with latest Swig or
Boost.Python.
But how to make it possible to do with Swig and Boost.Python together?

For example, we have two c++ extension A and B, which are exposed
with Swig and Boost.Python, respectively. They are as follows:

==============Module A wrapped with Swig ===============
   class Base
{
public:
        Base();
            ...
};
--------------------------------------------------------

========== Module B wrapped with Boost.Python ==========
void foo(Base *);

class Derive: public Base
{
        ...
};
--------------------------------------------------------

I wish to use module A and B in Python like this:

============= Module A and B used in Python ============
>>>import A
>>>import B
>>>obj = A.Base()
>>>B.foo(obj)
>>>obj = B.Derive()
>>>B.foo(obj)
--------------------------------------------------------

Could anybody give me a solution or some hints? I've working for
this for long.


On Fri, May 13, 2005 at 06:32:21PM -0700, Ralf W. Grosse-Kunstleve wrote:
> I guess the problem is that wxDC is not wrapped with Boost.Python, but with
> SWIG. Therefore Boost.Python doesn't know how to extract the wxDC instance (or
> a reference to the instance) from the Python object. I believe this can be

 Yes, wxPython uses SWIG.

> solved in a nice and general (via a custom from_python converter), but first
> I'd experiment like this (untested):
> 
>   void
>   foo(boost::python::object wxDC_swig_object)
>   {
>   }
> 
>   ...
> 
>   def("foo", foo);
> 
> Pass your SWIG-wrapped wxDC instance to foo(). This should work in any case, I
> hope. Once you get this working, develop the body of foo(). I'd look at the
> SWIG generated code for a wxPython function that accepts a wxDC object as an
> argument. SWIG uses PyArg_ParseTuple() to get a PyObject*; in foo() you want to
> use wxDC_swig_object.ptr() to get the same pointer. You have to find out how
> SWIG extracts the wxDC instance given the PyObject* and emulate this in the
> body of foo(). From within foo() you can finally call your function expecting a
> wxDC object.
> 
> Let us know if you get this far. The next step would be to rewrite foo() as a
> custom converter. However, it is only worth it if you want to wrap a number of
> functions with wxDC in the argument list. Otherwise the "thin wrapper" approach
> above (foo() is the thin wrapper) is the final solution.

Thanks very much. It's very useful for me. Your solution should be
correct, although I failed to test it for other reasons.

I've tried your approach, but it seems something wrong with my SWIG. 
SWIG's manual says I need link my extension with SWIG run time library 
(libswigpy.a or libswigpy.so). But in my installation of SWIG there
isn't nether libswigpy.a nor libswigpy.so. I don't know why.

> 
> --- Hua Su <suh at mails.tsinghua.edu.cn> wrote:
> > Hi,
> > 
> > I'v learnt Boost.Python for two weeks and now encounter some problems :( 
> > 
> >     In brief, I can't find the way to use a C++ class, which is exported in
> > an
> > 
> > existing extension, in another extension written by myself.
> > 
> >     In detail, I'm writing a paint program with wxPython which is an
> > 
> > extension for GUI library wxWidgets (wxWindows). I use wxPython to implement 
> > 
> > GUI, and use C++ to implement paint operations. 
> > 
> > ( In my application the paint operations is a bottleneck so I have to
> > implement
> >   
> >   is in C++ )
> > 
> > When painting, I need to pass a pointer of DC from Python to my C++ code.
> > 
> > The problem is, the class "wxDC" is a real C++ class writen in wxWidgets,
> > 
> > and is exported in wxPython as Python class "DC", but I need use it in my
> > 
> > code as C++ class "wxDC". I can't find support from Boost.Python for
> > 
> > this problem. I have tried to pass pointer "void *" instead, but Boost.Python
> > 
> > declares it does not support "void *" as argument or return type. 
> > 
> >   Could someone give me some example? I work for this problem for days :(
> > 
> > Thanks for your reply.
> > 
> > regards,
> > Clark
> 
> 
> 
> 		
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