test_untracked.cpp

#include <iostream>
#include <pybind11/pybind11.h>
namespace py = pybind11;

struct Foo { int v; Foo(int v) : v(v) {} };

PYBIND11_PLUGIN(test_untracked) {
    py::module m("test_untracked");
    py::class_<Foo>(m, "Foo")
        .def(py::init<int>());
    m.def("make_tracked", [](int i) -> Foo { return {i}; });
    m.def("make_untracked", [](int i) -> Foo { return {i}; },
          py::return_value_policy::copy_untracked);
    m.def("take_by_value", [](Foo f) { return f.v; });
    m.def("take_by_ref", [](Foo& f) { return f.v; });
    m.def("take_by_cref", [](const Foo& f) { return f.v; });
    return m.ptr();
}

test_untracked.py

from test_untracked import make_tracked, make_untracked, take_by_value, take_by_ref, take_by_cref

t = make_tracked(42)
assert take_by_value(t) == 42
assert take_by_ref(t) == 42
assert take_by_cref(t) == 42

u = make_untracked(2016)
assert take_by_value(u) == 2016
try:
    take_by_ref(u)
    raise Exception
except TypeError:
    pass
try:
    take_by_cref(u)
    raise Exception
except TypeError:
    pass