[BUG]: Incorrect values for first-several elements of Eigen::Vector when exposed with def_prop_ro

[zhipzhang]

Problem description

Description

I'm encountering an issue where the first few elements of an Eigen::VectorXf are incorrect when accessed from Python after being exposed as a read-only property using def_prop_ro. Specifically, the first four elements appear to have erroneous values. Using def to expose the same data as a method, or using std::vector instead of Eigen::VectorXf, works correctly. This suggests a potential incompatibility or bug related to how def_prop_ro handles Eigen::VectorXf.

The results are shown in the figure:

Reproducible example code

#include <nanobind/nanobind.h>
#include <nanobind/eigen/dense.h>
#include <Eigen/Dense>
#include <vector>

namespace nb = nanobind;

class test_Histogram2D {
public:
    test_Histogram2D(float min, float max, int bins) {
        bins_ = bins;
        min_ = min;
        max_ = max;
    }

    Eigen::VectorXf centers_eigen() const {
        Eigen::VectorXf centers(bins_);
        for (int i = 0; i < bins_; i++) {
            centers(i) = get_bin_center(i);
        }
        return centers;
    }

    std::vector<float> centers_stdvector() const {
        std::vector<float> centers(bins_);
        for (int i = 0; i < bins_; i++) {
            centers[i] = get_bin_center(i);
        }
        return centers;
    }

    float get_bin_center(int idx) const {
        return min_ + (idx + 0.5f) * (max_ - min_) / (float)bins_;
    }

private:
    int bins_;
    float min_;
    float max_;
};

NB_MODULE(example, m) {
    nb::class_<test_Histogram2D>(m, "test_Histogram2D")
        .def(nb::init<float, float, int>())
        .def_prop_ro("centers_eigen", &test_Histogram2D::centers_eigen)
        .def("centers_method", &test_Histogram2D::centers_eigen) // Working method version
        .def("centers_stdvector", &test_Histogram2D::centers_stdvector); //Also working
}