On 11/19/2009 07:30 AM, Mohan Ganesalingam wrote:
Dear all,
I recently sent an e-mail which (I think) didn't reach the list, about
trying to return large strings from C++ into Python without copying them.
I believe this is impossible: Python treats strings as immutable, which
means you can't manipulate them in-place, and there is no C API function
that allows you to pass in a string by reference, as that would make
memory management impossibly complex.
By looking at the C/Python API I've subsequently figured out roughly
how to do this... something like
Class World
{
object return_object()
{
PyObject *result = PyString_FromStringAndSize(0, 7);
This creates a new string, copying whatever you pass as input.
// initialise the underlying string using PyString_AsString
handle<> h(result);
return object();
I'm not sure what your intent is, but returning a default-constructed
object is the C++ equivalent of None.
}
...
main_namespace["World"] = class_<World>("World")
.def("return_object", &World::return_object);
Unfortunately this doesn't work; when I call w.return_object(), I get
back None.
...because above you "return object();", presumably.
Returning a raw PyObject * instead of a boost::python::object works,
but I can't tell if that involves a copy... and using
return_value_policy<manage_new_object>() with a PyObject * return
crashes.
Any advice here would be very much appreciated!
I don't think you can get what you want using strings directly. You may
create and expose a wrapper type that provides a string-like interface,
with appropriate conversions.
Stefan
--
...ich hab' noch einen Koffer in Berlin...
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