On 01/12/2011 09:03 AM, Matt Bendiksen wrote:
I'm stumped on a problem encountered when trying to return a derived instance
via a shared_ptr to python. The python instance is always shown as a base class
and not the subclass that was created, as if the instance was sliced.
struct A {
};
struct B : A {
};
shared_ptr<A> get_a_or_b_instance(bool returnB) {
return shared_ptr<A>(returnB ? new B() : new A());
}
…
def("get_a_or_b_instance", get_a_or_b_instance);
class_<A, boost::shared_ptr<A> >("A", init<>())
;
class_<B, bases<A>, boost::noncopyable>("B", init<>())
;
And the Python result is always a type A object:
>>> type(app.get_a_or_b_instance(False))
<class 'app.A'>
>>> type(app.get_a_or_b_instance(True))
<class 'app.A'>
Where I would expect the derived instance type B returned from the second call.
I'm using Boost 1.42.0.
Thanks in advance for any help.
You need to either declare B with
"class_<B, boost::shared_ptr<B>, bases<B>"
or do
"register_ptr_to_python< boost::shared_ptr<B> >()"
somewhere in your Python module. The latter will not cause B instances
to be stored in a shared_ptr when a Python constructor is called, but it
will still enable shared_ptr<B> downcasting on return.
Jim
Matt Bendiksen
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