how to keep_alive when returning a list or tuple?

[wojdyr]

Let's say that I have Items that reference Parent.

struct Item {
    Parent* parent;
};

Parent can return a vector with a number of such items

struct Parent {
    std::vector<Item> get_items() {
        return {Item{this}, Item{this}};
    }
};

All items should nurse their parent. If a single Item was returned, I'd just add keep_alive:

.def("get_single_item", &Parent::get_single_item, nb::keep_alive<0, 1>())

but if I try to do the same with a vector of items (the function get_items() defined above):

.def("get_items", &Parent::get_items, nb::keep_alive<0, 1>())

it complains that nanobind::detail::keep_alive(): could not create a weak reference! Likely, the 'nurse' argument you specified is not a weak-referenceable type!. (OK, the list can't be a nurse.)

How to solve it?

If I could use rv_policy::reference_internal, the policy would be passed to all elements and I think everything would just work. But in my case the items are not internal members. Is there a way to somehow pass keep_alive to all elements of a list?

[wjakob]

You're in luck: @oremanj added a new feature in the latest version of nanobind that I think could be used to support your use case: https://nanobind.readthedocs.io/en/latest/api_core.html#_CPPv4I0EN8nanobind11call_policyE

[wojdyr]

Great! Thanks @oremanj for solving my problem before I asked.
The example in docs is exactly what I needed, with a minor fix: I > 0 && I < N seems to be off by one. I think it meant to be <= N and then args[I-1] when calling detail::keep_alive.