How to use .cast() with a signature of a function?

[rmoyard]

Hello!

I am trying to return an array allocated on stack and was following the guide https://nanobind.readthedocs.io/en/latest/ndarray.html#returning-temporaries. I am encountering the following error, where it states what is is returned does not match the function signature.

nb::ndarray<nb::numpy, double, nb::shape<-1>, nb::device::cpu> my_function(...)
    size_t total_size = (2 * ... + 1);
    double results[total_size];
    ....
    return nb::ndarray<nb::numpy, double, nb::shape<-1>, nb::device::cpu>(results, {total_size}).cast();

Could someone tell me what to do in that case? It works without the .cast but it is bad practice for stack allocated variables.

error: could not convert ‘nanobind::ndarray<Args>::cast(nanobind::rv_policy, nanobind::handle) [with Args = {nanobind::numpy, double, nanobind::detail::shape<-1>, nanobind::device::cpu}](nanobind::rv_policy::automatic, nanobind::handle())’ from ‘nanobind::detail::ndarray_object<nanobind::numpy, double, nanobind::detail::shape<-1>, nanobind::device::cpu>’ to ‘nanobind::ndarray<nanobind::numpy, double, nanobind::detail::shape<-1>, nanobind::device::cpu>’
  238 |    return nb::ndarray<nb::numpy, double, nb::shape<-1>, nb::device::cpu>(results, {total_size}).cast();
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
      |                                                                                                     |
      |                                                                                                     nanobind::detail::ndarray_object<nanobind::numpy, double, nanobind::detail::shape<-1>, nanobind::device::cpu>

[oremanj]

ndarray::cast() returns an instance of nb::detail::ndarray_object<Args...>, not nb::ndarray<Args...>. They are not interconvertible; ndarray_object wraps a Python object while ndarray doesn't necessarily. It is probably ideal to write your function with an auto return type in order to avoid needing to name this internal type.

[rmoyard]

Thanks!