Demo: return_value_policy::copy ignored

tests/CMakeLists.txt

@@ -147,6 +147,7 @@ set(PYBIND11_TEST_FILES
     test_opaque_types
     test_operator_overloading
     test_pickling
+    test_property_stl
     test_pytypes
     test_sequences_and_iterators
     test_smart_ptr

tests/test_property_stl.cpp

@@ -0,0 +1,45 @@
+#include <pybind11/stl.h>
+
+#include "pybind11_tests.h"
+
+#include <cstddef>
+#include <vector>
+
+namespace test_property_stl {
+
+struct Field {
+    explicit Field(int wrapped_int) : wrapped_int{wrapped_int} {}
+    int wrapped_int = 100;
+};
+
+struct FieldHolder {
+    explicit FieldHolder(const Field &fld) : fld{fld} {}
+    Field fld = Field{200};
+};
+
+struct VectorFieldHolder {
+    std::vector<FieldHolder> vec_fld_hld;
+    VectorFieldHolder() { vec_fld_hld.push_back(FieldHolder{Field{300}}); }
+    void reset_at(std::size_t index, int wrapped_int) {
+        if (index < vec_fld_hld.size()) {
+            vec_fld_hld[index].fld.wrapped_int = wrapped_int;
+        }
+    }
+};
+
+} // namespace test_property_stl
+
+TEST_SUBMODULE(property_stl, m) {
+    using namespace test_property_stl;
+
+    py::class_<Field>(m, "Field").def_readwrite("wrapped_int", &Field::wrapped_int);
+
+    py::class_<FieldHolder>(m, "FieldHolder").def_readwrite("fld", &FieldHolder::fld);
+
+    py::class_<VectorFieldHolder>(m, "VectorFieldHolder")
+        .def(py::init<>())
+        .def("reset_at", &VectorFieldHolder::reset_at)
+        .def_readwrite("vec_fld_hld_ref", &VectorFieldHolder::vec_fld_hld)
+        .def_readwrite(
+            "vec_fld_hld_cpy", &VectorFieldHolder::vec_fld_hld, py::return_value_policy::copy);
+}

tests/test_property_stl.py

@@ -0,0 +1,31 @@
+from pybind11_tests import property_stl as m
+
+
+def test_cpy_after_ref():
+    h = m.VectorFieldHolder()
+    c1 = h.vec_fld_hld_cpy
+    c2 = h.vec_fld_hld_cpy
+    assert repr(c2) != repr(c1)
+    r1 = h.vec_fld_hld_ref
+    assert repr(r1) != repr(c2)
+    assert repr(r1) != repr(c1)
+    r2 = h.vec_fld_hld_ref
+    assert repr(r2) == repr(r1)
+    c3 = h.vec_fld_hld_cpy
+    assert repr(c3) == repr(r1)  # SURPRISE!
+
+
+def test_persistent_holder():
+    h = m.VectorFieldHolder()
+    c0 = h.vec_fld_hld_cpy[0]  # Must be first. See test_cpy_after_ref().
+    r0 = h.vec_fld_hld_ref[0]  # Must be second.
+    assert c0.fld.wrapped_int == 300
+    assert r0.fld.wrapped_int == 300
+    h.reset_at(0, 400)
+    assert c0.fld.wrapped_int == 300
+    assert r0.fld.wrapped_int == 400
+
+
+def test_temporary_holder_keep_alive():
+    r0 = m.VectorFieldHolder().vec_fld_hld_ref[0]
+    assert r0.fld.wrapped_int == 300