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