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

Ralf W. Grosse-Kunstleve rwgk at yahoo.com
Sun Jul 3 20:56:12 CEST 2005

--- David Abrahams <dave at boost-consulting.com> wrote:
> > Looking some more, isn't this almost it?
> >
> >   http://www.boost.org/libs/python/doc/v2/lvalue_from_pytype.html
> Yes.


> > just knowing that ((PySwigObject*)obj.ptr())->ptr gives us the T*.
> Oh, I see; all Swig objects have the same type?


> Well you need some way to identify the Python objects for which the
> above statement is true, unless you want to forego all error checking
> and just allow a crash if a Swig-wrapped Foo is passed where a
> Swig-wrapped Bar is expected.

I hadn't though of that. Thanks, David!

Encouraged by your "Yes" above I gave it a good push. Here is the


  (Also attached.)

This solution is based on what I found in SWIG-1.3.24/Lib/python/pyrun.swg.
At the heart of the solution is this simple fragment:

    static void* extract(PyObject* op) \
    { \
        if (std::strcmp(op->ob_type->tp_name, "PySwigObject") != 0) return 0; \
        PySwigObject* swig_obj_ptr = reinterpret_cast<PySwigObject*>(op); \
        if (std::strcmp(swig_obj_ptr->desc, "_p_" # T) != 0) return 0; \
        return swig_obj_ptr->ptr; \
    } \

And here is the corresponding test script:


The test script slightly edited:

    import example # SWIG-1.3.24/Examples/python/class
    import boost_python_swig_args_ext

    c = example.Circle(10)
    c.x = 20
    c.y = 30

    s = example.Square(10)
    s.x = -10
    s.y = 5


The test runs successfully under Linux. It proves that the overloaded
show() function works correctly, which means the simple strcmp() logic
in the C++ fragment above works as it should. I also used valgrind and
did a leak check. If someone wants to reproduce:

    gunzip -c swig-1.3.24.tar.gz | tar xf -
    cd SWIG-1.3.24
    cd Examples/python/class
    cp _example.so example.py <somewhere on PYTHONPATH>
    ar r libswig_class_example.a example.o

Then compile and link boost_python_swig_args_ext with

David, would you want to include the core of swig_args_ext.cpp in,
e.g., boost/python/swig_arg.h? I think it would be a valuable
addition. There are many SWIG-wrapped libraries. People could easily
use them while writing their own extensions with Boost.Python.


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