[C++-sig] Wrapping opaque pointers returned from API functions

[Gottfried.Ganssauge at HAUFE.DE]

I am trying to wrap an API like the following:
	typedef struct workspace_ *workspace;

	workspace osr_init (const char *) { return 0; }
	void osr_exit (workspace ws) { std::cerr << ws << std::endl; }

My first attempt at wrapping that API was
	BOOST_PYTHON_MODULE(_pyOSR)
	{
		def ("osr_init", &::osr_init,
return_value_policy<return_by_value>());
		def ("osr_exit", &::osr_exit);
	}

That compiled fine, but (of course) failed miserably when run with the
following test script:
	import _pyOSR

	ws = _pyOSR.osr_init(".")
	print ws
	_pyOSR.osr_exit(ws)

Here is the traceback:
Traceback (most recent call last):
  File
"C:\daten\devel\sourcecode-cd\OptiSearch\src\pyOSR\msvc\Release\test.py",
 line 3, in ?
    ws = _pyOSR.osr_init(".")
TypeError: No to_python (by-value) converter found for C++ type: struct
workspace_ *

Now I thought: ok, let's register a converter
    extern "C" void
    dealloc(PyObject* self) {
        PyObject_Del(self);
    }

    struct workspace_wrapper : PyObject { workspace x; };

    PyTypeObject workspaceType = {
        PyObject_HEAD_INIT(NULL)
        0,
        "workspace",
        sizeof(workspace_wrapper),
        0,
        dealloc
    };

    struct p_from_workspace {
        static workspace &execute (workspace_wrapper &p_) {
            return p_.x;
        }
    };

    struct pworkspace_to_python
        : to_python_converter<workspace, pworkspace_to_python>
    {
        static PyObject* convert(workspace x) {
            workspace_wrapper *o =
                PyObject_New (workspace_wrapper, &workspaceType);
            o->x = x;
            return ((PyObject*) o);
        }
    };

    BOOST_PYTHON_MODULE(_pyOSR)
    {
        pworkspace_to_python();
        lvalue_from_pytype<p_from_workspace, &workspaceType>();

        def ("osr_init", &::osr_init,
return_value_policy<return_by_value>());
        def ("osr_exit", &::osr_exit);
    }

This time the output changed:
<workspace object at 0x00768EA0>
Traceback (most recent call last):
  File
"C:\daten\devel\sourcecode-cd\OptiSearch\src\pyOSR\msvc\Release\test.py",
 line 5, in ?
    _pyOSR.osr_exit(ws)
TypeError: bad argument type for built-in operation

The call to osr_init() succeeded and produced the expected result: a
workspace object.
Unfortunately the library is unable to find the proper converter for
converting that object back to my workspace pointer. I'm sure, there is
something obvious I'm doing wrong, but what?

Cheers,
 
Gottfried Ganßauge
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