Making smart pointers a bit smarter

Hi folks,

SWIG  (at least swig -python) has nice support for wrapping C++ smart
pointers.  One thing that I feel could be improved is the handling of
NULL smart pointers.  When converting a NULL C++ raw pointer to
python, SWIG (correctly, in my view) converts it to None.  However,
SWIIG doesn't do anything similar for smart pointers --- it basically
wraps the NULL smart pointer and returns it.  A NULL smart pointer
wrapped this way is a booby trap --- any attempt to use it for
anything causes a SEGV.

More specifically, the default typemap results in the following code
to return a smart pointer to python:

  {
      FooSmartptr * resultptr;
      resultptr = new FooSmartptr((FooSmartptr &) result);
      resultobj = SWIG_NewPointerObj((void *) resultptr,
SWIGTYPE_p_FooSmartptr, 1);
  }

With the following typemap, the behavior can be improved (in my
opinion) so that None is returned if the smart pointer is NULL:

%typemap(out) FooSmartptr {
 if( $1.operator->() ) {
    FooSmartptr * resultptr;
    resultptr = new FooSmartptr( $1 );
    $result = SWIG_NewPointerObj( resultptr, SWIGTYPE_p_FooSmartptr, 1 );
 } else {
    Py_INCREF(Py_None);
    $result = Py_None;
 }
}

"None" is much friendlier than a wrapped NULL smart pointer --- it's
easy to test for "None", and if you try to use it as a pointer, you
get a sensible Python exception (rather than a SEGV).

All of this is fine --- the reason I'm writing is I would like to make
this improved smart pointer wrapping the default behavior of SWIG.
Before I spend the time to do so, I wanted to guage the interest level
from the SWIG maintainers --- is the patch likely to be accepted
(assuming I code it competently), or is it something you're not
interested in (i.e., is there a good reason to not do this)?

Thanks,
  -Ken
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