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Later assignments to accessors should not overwrite the original field
`auto var = l[0]` has a strange quirk: `var` is actually an accessor and not an object, so any later assignment of `var = ...` would modify l[0] instead of `var`. This is surprising compared to the non-auto assignment `py::object var = l[0]; var = ...`. By overloading `operator=` on lvalue/rvalue, the expected behavior is restored even for `auto` variables.
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@ -18,6 +18,7 @@
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# pragma warning(disable: 4800) // warning C4800: 'int': forcing value to bool 'true' or 'false' (performance warning)
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# pragma warning(disable: 4996) // warning C4996: The POSIX name for this item is deprecated. Instead, use the ISO C and C++ conformant name
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# pragma warning(disable: 4702) // warning C4702: unreachable code
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# pragma warning(disable: 4522) // warning C4522: multiple assignment operators specified
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#elif defined(__INTEL_COMPILER)
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# pragma warning(push)
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# pragma warning(disable: 186) // pointless comparison of unsigned integer with zero
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@ -199,16 +199,19 @@ class accessor : public object_api<accessor<Policy>> {
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public:
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accessor(handle obj, key_type key) : obj(obj), key(std::move(key)) { }
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void operator=(const accessor &a) { operator=(handle(a)); }
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void operator=(const object &o) { operator=(handle(o)); }
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void operator=(handle value) { Policy::set(obj, key, value); }
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void operator=(const accessor &a) && { std::move(*this).operator=(handle(a)); }
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void operator=(const accessor &a) & { operator=(handle(a)); }
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void operator=(const object &o) && { std::move(*this).operator=(handle(o)); }
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void operator=(const object &o) & { operator=(handle(o)); }
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void operator=(handle value) && { Policy::set(obj, key, value); }
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void operator=(handle value) & { get_cache() = object(value, true); }
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operator object() const { return get_cache(); }
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PyObject *ptr() const { return get_cache().ptr(); }
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template <typename T> T cast() const { return get_cache().template cast<T>(); }
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private:
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const object &get_cache() const {
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object &get_cache() const {
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if (!cache) { cache = Policy::get(obj, key); }
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return cache;
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}
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@ -272,4 +272,21 @@ test_initializer python_types([](py::module &m) {
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}
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return py::tuple();
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});
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m.def("test_accessor_assignment", []() {
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auto l = py::list(1);
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l[0] = py::cast(0);
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auto d = py::dict();
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d["get"] = l[0];
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auto var = l[0];
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d["deferred_get"] = var;
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l[0] = py::cast(1);
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d["set"] = l[0];
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var = py::cast(99); // this assignment should not overwrite l[0]
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d["deferred_set"] = l[0];
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d["var"] = var;
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return d;
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});
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});
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@ -251,7 +251,7 @@ def test_dict_api():
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def test_accessors():
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from pybind11_tests import test_accessor_api, test_tuple_accessor
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from pybind11_tests import test_accessor_api, test_tuple_accessor, test_accessor_assignment
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class SubTestObject:
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attr_obj = 1
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@ -280,3 +280,10 @@ def test_accessors():
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assert d["operator*"] == 7
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assert test_tuple_accessor(tuple()) == (0, 1, 2)
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d = test_accessor_assignment()
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assert d["get"] == 0
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assert d["deferred_get"] == 0
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assert d["set"] == 1
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assert d["deferred_set"] == 1
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assert d["var"] == 99
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