tests/test_potentially_slicing_weak_ptr.cpp

// Copyright (c) 2025 The pybind Community.
// All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "pybind11_tests.h"

#include <memory>

namespace pybind11_tests {
namespace potentially_slicing_weak_ptr {

template <int> // Using int as a trick to easily generate multiple types.
struct VirtBase {
    VirtBase() = default;
    virtual ~VirtBase() = default;
    VirtBase(const VirtBase &) = delete;
    virtual int get_code() { return 100; }
};

using VirtBaseSH = VirtBase<0>; // for testing with py::smart_holder
using VirtBaseSP = VirtBase<1>; // for testing with std::shared_ptr as holder

// Similar to trampoline_self_life_support
struct trampoline_is_alive_simple {
    std::uint64_t magic_token = 197001010000u;

    trampoline_is_alive_simple() = default;

    ~trampoline_is_alive_simple() { magic_token = 20380118191407u; }

    trampoline_is_alive_simple(const trampoline_is_alive_simple &other) = default;
    trampoline_is_alive_simple(trampoline_is_alive_simple &&other) noexcept
        : magic_token(other.magic_token) {
        other.magic_token = 20380118191407u;
    }

    trampoline_is_alive_simple &operator=(const trampoline_is_alive_simple &) = delete;
    trampoline_is_alive_simple &operator=(trampoline_is_alive_simple &&) = delete;
};

template <typename VB>
const char *determine_trampoline_state(const std::shared_ptr<VB> &sp) {
    if (!sp) {
        return "sp nullptr";
    }
    auto *tias = dynamic_cast<trampoline_is_alive_simple *>(sp.get());
    if (!tias) {
        return "dynamic_cast failed";
    }
    if (tias->magic_token == 197001010000u) {
        return "trampoline alive";
    }
    if (tias->magic_token == 20380118191407u) {
        return "trampoline DEAD";
    }
    return "UNDEFINED BEHAVIOR";
}

struct PyVirtBaseSH : VirtBaseSH, py::trampoline_self_life_support, trampoline_is_alive_simple {
    using VirtBaseSH::VirtBaseSH;
    int get_code() override { PYBIND11_OVERRIDE(int, VirtBaseSH, get_code); }
};

struct PyVirtBaseSP : VirtBaseSP, trampoline_is_alive_simple { // self-life-support not available
    using VirtBaseSP::VirtBaseSP;
    int get_code() override { PYBIND11_OVERRIDE(int, VirtBaseSP, get_code); }
};

template <typename VB>
std::shared_ptr<VB> rtrn_obj_cast_shared_ptr(py::handle obj) {
    return obj.cast<std::shared_ptr<VB>>();
}

// There is no type_caster<std::weak_ptr<VB>>, and to minimize code complexity
// we do not want to add one, therefore we have to return a shared_ptr here.
template <typename VB>
std::shared_ptr<VB> rtrn_potentially_slicing_shared_ptr(py::handle obj) {
    return py::potentially_slicing_weak_ptr<VB>(obj).lock();
}

template <typename VB>
struct SpOwner {
    void set_sp(const std::shared_ptr<VB> &sp_) { sp = sp_; }

    int get_code() const {
        if (!sp) {
            return -888;
        }
        return sp->get_code();
    }

    const char *get_trampoline_state() const { return determine_trampoline_state(sp); }

private:
    std::shared_ptr<VB> sp;
};

template <typename VB>
struct WpOwner {
    void set_wp(const std::weak_ptr<VB> &wp_) { wp = wp_; }

    int get_code() const {
        auto sp = wp.lock();
        if (!sp) {
            return -999;
        }
        return sp->get_code();
    }

    const char *get_trampoline_state() const { return determine_trampoline_state(wp.lock()); }

private:
    std::weak_ptr<VB> wp;
};

template <typename VB>
void wrap(py::module_ &m,
          const char *roc_pyname,
          const char *rps_pyname,
          const char *spo_pyname,
          const char *wpo_pyname) {
    m.def(roc_pyname, rtrn_obj_cast_shared_ptr<VB>);
    m.def(rps_pyname, rtrn_potentially_slicing_shared_ptr<VB>);

    py::classh<SpOwner<VB>>(m, spo_pyname)
        .def(py::init<>())
        .def("set_sp", &SpOwner<VB>::set_sp)
        .def("get_code", &SpOwner<VB>::get_code)
        .def("get_trampoline_state", &SpOwner<VB>::get_trampoline_state);

    py::classh<WpOwner<VB>>(m, wpo_pyname)
        .def(py::init<>())
        .def("set_wp",
             [](WpOwner<VB> &self, py::handle obj) {
                 self.set_wp(obj.cast<std::shared_ptr<VB>>());
             })
        .def("set_wp_potentially_slicing",
             [](WpOwner<VB> &self, py::handle obj) {
                 self.set_wp(py::potentially_slicing_weak_ptr<VB>(obj));
             })
        .def("get_code", &WpOwner<VB>::get_code)
        .def("get_trampoline_state", &WpOwner<VB>::get_trampoline_state);
}

} // namespace potentially_slicing_weak_ptr
} // namespace pybind11_tests

using namespace pybind11_tests::potentially_slicing_weak_ptr;

TEST_SUBMODULE(potentially_slicing_weak_ptr, m) {
    py::classh<VirtBaseSH, PyVirtBaseSH>(m, "VirtBaseSH")
        .def(py::init<>())
        .def("get_code", &VirtBaseSH::get_code);

    py::class_<VirtBaseSP, std::shared_ptr<VirtBaseSP>, PyVirtBaseSP>(m, "VirtBaseSP")
        .def(py::init<>())
        .def("get_code", &VirtBaseSP::get_code);

    wrap<VirtBaseSH>(m,
                     "SH_rtrn_obj_cast_shared_ptr",
                     "SH_rtrn_potentially_slicing_shared_ptr",
                     "SH_SpOwner",
                     "SH_WpOwner");

    wrap<VirtBaseSP>(m,
                     "SP_rtrn_obj_cast_shared_ptr",
                     "SP_rtrn_potentially_slicing_shared_ptr",
                     "SP_SpOwner",
                     "SP_WpOwner");
}