Using Eigen objects as properties (nOOb question)

[tvatter]

Hi,

Mostly for performance reasons, I am porting pyvinecopulib from pybind11 to nanobind. In pybind11, I used def_property and def_property_readonly for private internal variables (Eigen objects) that can only be accessed via setters and getters. I tried to do the same with nanobind, and an MWE is below:

#include <nanobind/nanobind.h>
#include <nanobind/eigen/dense.h>

namespace nb = nanobind;

using namespace nb::literals;

class EigenVector
{

private:
  Eigen::VectorXd vec;

public:
  EigenVector() : vec(Eigen::VectorXd::Zero(3)) {}

  EigenVector(const Eigen::VectorXd &vec) : vec(vec) {}

  Eigen::VectorXd get() const
  {
    return vec;
  }

  void set(const Eigen::VectorXd &vec)
  {
    this->vec = vec;
  }
};

NB_MODULE(nanobind_example_ext, m) {

  m.doc() = "An example with Eigen vectors";

  nb::class_<EigenVector>(m, "EigenVector")
      .def(nb::init<>())
      .def(nb::init<const Eigen::VectorXd &>(), "vec"_a)
      .def_prop_rw("vec", &EigenVector::get, &EigenVector::set)
      .def_prop_ro("vec_ro", &EigenVector::get)
      .def("vec_def", &EigenVector::get)
      .def_prop_ro("vec_ro_nbcast", [](const EigenVector &v)
                   { return nb::cast(v.get()); })
      .def_prop_rw("vec_rw_nbcast", [](EigenVector &v)
                   { return nb::cast(v.get()); }, &EigenVector::set);
}

However, when trying to use this in Python, something weird happens:

>>> import numpy as np
>>> import nanobind_example as m
>>> v = np.array([1, 2, 3])
>>> v2 = m.EigenVector(v)
>>> print(v2.vec)
[5.50215936e-310 0.00000000e+000 3.00000000e+000]
>>> print(v2.vec_ro)
[ 5.50216011e-310 -1.84063345e+286  3.00000000e+000]
>>> print(v2.vec_def())
[1. 2. 3.]
>>> print(v2.vec_ro_nbcast)
[1. 2. 3.]
>>> print(v2.vec_rw_nbcast)
[1. 2. 3.]

I am sure that there's something that I don't understand with how objects are passed between C++ and Python. I've read attentively nanobind's related doc page but for the life of me, I don't understand what I'm doing wrong. Why are we supposed to cast the output of the getter in this way?