Custom constructor with arguments?

[thomthom]

I'm trying to port my library from pybind11 to nanobind. Not sure how to deal with a custom constructor I have that takes parameters:

  nb::class_<fastgpx::Bounds>(m, "Bounds")
      .def(nb::init<>())
      .def(nb::init<const fastgpx::LatLong&, const fastgpx::LatLong&>(), nb::arg("min"),
           nb::arg("max"))
      // Allow tuples instead of explicit LatLong objects.
      .def(nb::init([](std::tuple<double, double> min_tuple, std::tuple<double, double> max_tuple) {
             fastgpx::LatLong min{std::get<0>(min_tuple), std::get<1>(min_tuple)};
             fastgpx::LatLong max{std::get<0>(max_tuple), std::get<1>(max_tuple)};
             return fastgpx::Bounds(min, max);
           }),
           nb::arg("min"), nb::arg("max"))
      .def_rw("min", &fastgpx::Bounds::min)
      .def_rw("max", &fastgpx::Bounds::max)
      .def("is_empty", &fastgpx::Bounds::IsEmpty)

The porting guide shows an example, but without arguments? From what I understand, when you create a custom ctor with nanobind you're given a pointer to some memory where you can use placement new. But where do you obtain arguments given to the custom ctor?

[thomthom]

I think I got it. The pre-allocated comes first, then any arguments comes afterwards:

      // Allow tuples instead of explicit LatLong objects.
      .def(
          "__init__",
          [](fastgpx::Bounds* obj, std::tuple<double, double> min_tuple,
             std::tuple<double, double> max_tuple) {
            fastgpx::LatLong min{std::get<0>(min_tuple), std::get<1>(min_tuple)};
            fastgpx::LatLong max{std::get<0>(max_tuple), std::get<1>(max_tuple)};
            return new (obj) fastgpx::Bounds(min, max);
          },
          nb::arg("min"), nb::arg("max"))

[thomthom]

That worked, but the stub generator ends up putting the return type of the lambda into the signature, even though this is an __init__ method that should only return None:

class Bounds:
    @overload
    def __init__(self) -> None: ...

    @overload
    def __init__(self, min: LatLong, max: LatLong) -> None: ...

    @overload
    def __init__(self, min: tuple[float, float], max: tuple[float, float]) -> Bounds: ...

I guess one workaround is to use .sig to correct that. But maybe the real fix would be for the stub generator to always emit None for the return type of __init__ methods?

[wjakob]

Remove the return statement from your lambda function.

[thomthom]

Ah! That did the trick - thank you!