pybind11/tests/test_private_first_base.cpp

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// Demonstration of UB (Undefined Behavior) in handling of polymorphic pointers,
// specifically:
// https://github.com/pybind/pybind11/blob/30eb39ed79d1e2eeff15219ac00773034300a5e6/include/pybind11/cast.h#L229
// `return reinterpret_cast<V *&>(vh[0]);`
// casts a `void` pointer to a `base`. The `void` pointer is obtained through
// a `dynamic_cast` here:
// https://github.com/pybind/pybind11/blob/30eb39ed79d1e2eeff15219ac00773034300a5e6/include/pybind11/cast.h#L852
// `return dynamic_cast<const void*>(src);`
// The `dynamic_cast` is well-defined:
// https://en.cppreference.com/w/cpp/language/dynamic_cast
// 4) If expression is a pointer to a polymorphic type, and new-type
// is a pointer to void, the result is a pointer to the most derived
// object pointed or referenced by expression.
// But the `reinterpret_cast` above is UB: `test_make_drvd_pass_base` in
// `test_private_first_base.py` fails with a Segmentation Fault (Linux,
// clang++ -std=c++17).
// The only well-defined cast is back to a `drvd` pointer (`static_cast` can be
// used), which can then safely be cast up to a `base` pointer. Note that
// `test_make_drvd_up_cast_pass_drvd` passes because the `void` pointer is cast
// to `drvd` pointer in this situation.
#include "pybind11_tests.h"
namespace pybind11_tests {
namespace private_first_base {
struct base {
base() : base_id(100) {}
virtual ~base() = default;
virtual int id() const { return base_id; }
base(const base&) = delete;
int base_id;
};
struct private_first_base { // Any class with a virtual function will do.
virtual void some_other_virtual_function() const {}
virtual ~private_first_base() = default;
};
struct drvd : private private_first_base, public base {
int id() const override { return 2 * base_id; }
};
inline drvd* make_drvd() { return new drvd; }
inline base* make_drvd_up_cast() { return new drvd; }
inline int pass_base(const base* b) { return b->id(); }
inline int pass_drvd(const drvd* d) { return d->id(); }
TEST_SUBMODULE(private_first_base, m) {
py::class_<base>(m, "base");
py::class_<drvd, base>(m, "drvd");
m.def("make_drvd", make_drvd,
py::return_value_policy::take_ownership);
m.def("make_drvd_up_cast", make_drvd_up_cast,
py::return_value_policy::take_ownership);
m.def("pass_base", pass_base);
m.def("pass_drvd", pass_drvd);
}
} // namespace private_first_base
} // namespace pybind11_tests