[C++-sig] Re: creating an instance of a class_<> object in c++ and exporting it to python
ostiguy at fnal.gov
Wed Aug 18 21:01:50 CEST 2004
> That's not a wrapper class; it's a smart pointer.
I apologize for the the poor wording. One might argue that the
particular smart pointer in my example is a "wrapper" around a regular
pointer. I agree that "wrapper class", especially in the context of
the boost.python library, has a more precise meaning.
Let me try to ask my questions differently.
Your explanation of how the conversion is handled is
"So the conversion is
handled by the make_function call that created py_get_existing.
However, you need class_<A...> in order to create the Python class
object corresponding to A."
When make_function() converts a c++ A* into a python object "A",
(1) knowing what the python type A interface is, which is why class_<A,..>
(2) deciding what will be held in that object. In my case, the return
value policy implies that I want the object to hold a dumb
What determines the valid held types ? Do I understand correctly that A*, A&,
auto_ptr<A> are automatically valid held types and therefore explicitly specifying
A* is uncessary ? In other words, the only held types that need to be explicitly
specified are those that are not already supported ?
When I do
class<A, boost:noncopyable>("A", no_init);
class<B,SmartPointer<B>, boost:noncopyable>("B", no_init);
object PythonInstanceofA ( a ); // error 
object PythonInstanceofB ( SmartPointer<B>(b) ); // OK 
according to your previous explanations:
 "fails because there is an attempt to copy "a" directly into the
A object held by PythonInstanceofA"
object is generic object. By what mechanism would it know prior
to construction that it should be holding an instance of A ?
I would think that PythonInstanceofA can only acquire its identity as a
type "A" by infering it from the argument in the constructor. So I do not
understand the statement "copy "a" directly into the A object held by
 "succeeds because Boost.Python recognizes that it's a wrapper class
that contains a PyObject* referring to a Python object, and
just extracts that pointer"
Which "wrapper" class are you refering to ? SmartPointer<T> ?
Does class<B, SmartPointer<B>, boost:noncopyable> register a
converter from SmartPointer<B> to Python ?
Thank you in advance. Your patience is greatly appreciated ;-)
Dr. Jean-Francois OSTIGUY voice: (630) 840-2231
Beam Physics Dept MS220 FAX: (630) 840-6039
Fermi National Accelerator Laboratory email: ostiguy at fnal.gov
Batavia IL 60510-0500 WWW:www-ap.fnal.gov/~ostiguy
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