Returning a vector of pointers managed on the C++ side

[thelfer]

Dear nanobind community,

My question is about wrapping a method returning a vector of pointer. The pointers are owned by the C++ objects.

To illustrate the point, I have something like:

struct Element;

struct ElementsManager{
std::vector<Element *>  select(std::string view);
private:
std::vector<std::unique_ptr<Element>> elements;
};

How can I wrap the select method ? Of course, the better would be to use the automatic type conversion provided by the type casters given in nanobind/stl/vector.h so that python would see a list of Elements.

Thanks for any help.

[thelfer]

up ?

[wjakob]

I would say: use the normal std::vector type caster, but bind the method with a return value policy of reference_internal.

(You'll want to read up on RVPs in the documentation if this doesn't make sense)

[thelfer]

Thank for the answer. So the idea would be to simply:

nb::class_<ElementsManager>(m, "ElementsManager")
  .def("select", &ElementsManager::select, nb::rv_policy::reference_internal);

The RVP would apply to all elements of the vector ? That's perfect.

Maybe you would be interested by a small comment from a new comer to nanobind.

I did not find this behavior described in the documentation (of course I may have missed it in some advanced pages, which I also read, but there is nothing about it in the two pages cited below).

This behavior is a bit counter-intuitive at first: I would expect the RVP to apply on the returned vector, not to the hold elements.

Maybe a small description derived from this example in the RVP section (https://nanobind.readthedocs.io/en/latest/ownership.html) and/or in the section relative to the type_caster (https://nanobind.readthedocs.io/en/latest/exchanging.html) would be beneficial.

Of course, that's only a suggestion.