Unique pointer

[petrasvestartas]

I am using unique pointers in nanobind, without thinking that I need to delete them. Now I am reading this section of documentation:
https://nanobind.readthedocs.io/en/latest/ownership.html

Questions:

1. Does the consume method transfers the object from python to c++ where it is eventually destroyed?
2. If I do not use consume method and the object instance stays in python what could happen?
3. Can I transfer this Data instance as unique pointer to C++ without deleting it? E.g. to sum-up 2 numbers from the 2 class instances attributes.

#include <nanobind/stl/unique_ptr.h>

namespace nb = nanobind;

NB_MODULE(my_ext, m) {
    struct Data { };
    nb::class_<Data>(m, "Data");
    m.def("create", []() { return std::make_unique<Data>(); });
    m.def("consume", [](std::unique_ptr<Data> x) { /* no-op */ });
}

import my_ext

x = my_ext.create()
my_ext.consume(x)

[wjakob]

1. yes
2. Python will eventually delete the instance when it is garbage collected
3. You can make a function that just takes a regular pointer argument. Passing std::unique_ptr always implies a transfer of ownership. I guess you could return the value again, but that makes for a clunky API.

Based on your question in 3, I am wondering if what you really want is unique pointers. Shared pointers or intrusive reference counting might be a better ownership model.

[petrasvestartas]

How complicated would be to create a custom pointer?

I am using OCCT library that has its own shared pointer handling mechanism:
https://github.com/Open-Cascade-SAS/OCCT/blob/master/src/FoundationClasses/TKernel/Standard/Standard_Handle.hxx

This is how it is normally used:

Handle(Geom_BSplineSurface) surface = nurbs_surface_from_points(points_matrix, rows, cols, degree_u, degree_v);

With nanobind I do the following

std::unique_ptr<NurbsSurfaceData> create_nurbs_surface_from_points(
    const nb::DRef<Eigen::MatrixXd>& points_matrix,
    int rows, int cols,
    int degree_u = 3,
    int degree_v = 3
) {
    Handle(Geom_BSplineSurface) surface = nurbs_surface_from_points(points_matrix, rows, cols, degree_u, degree_v);
    return extract_nurbs_surface_data(surface);
}

It feels that it is a pointer on top of a pointer.
Do you think it would be worth to invest time for this? Or better to use standard library unique or shared pointers?