show object slicing w/ pickling C++ derived object not visible to pybind

Author: EricCousineau-TRI

tests/test_pickling.cpp

@@ -26,11 +26,19 @@ struct SimpleBase {
     // For compatibility with old clang versions:
     SimpleBase()                   = default;
     SimpleBase(const SimpleBase &) = default;
+
+    virtual int get_value_via_vtable() const { return 1; }
 };
 
-struct SimpleBaseTrampoline : SimpleBase {};
+struct SimpleBaseTrampoline : SimpleBase {
+    int get_value_via_vtable() const override {
+        PYBIND11_OVERRIDE(int, SimpleBase, get_value_via_vtable);
+    }
+};
 
-struct SimpleCppDerived : SimpleBase {};
+struct SimpleCppDerived : SimpleBase {
+    int get_value_via_vtable() const override { return 10; }
+};
 
 void wrap(py::module m) {
     py::class_<SimpleBase, SimpleBaseTrampoline>(m, "SimpleBase")
@@ -54,6 +62,9 @@ void wrap(py::module m) {
 
     m.def("make_SimpleCppDerivedAsBase",
           []() { return std::unique_ptr<SimpleBase>(new SimpleCppDerived); });
+
+    m.def("get_value_via_vtable_cpp",
+          [](const SimpleBase& obj) { return obj.get_value_via_vtable(); });
 }
 
 } // namespace exercise_trampoline

tests/test_pickling.py

@@ -51,23 +51,28 @@ def test_enum_pickle():
 # exercise_trampoline
 #
 class SimplePyDerived(m.SimpleBase):
-    pass
+    def get_value_via_vtable(self):
+        return 100
+
 
 
 def test_roundtrip_simple_py_derived():
     p = SimplePyDerived()
     p.num = 202
     p.stored_in_dict = 303
+    assert m.get_value_via_vtable_cpp(p) == 100
     data = pickle.dumps(p, pickle.HIGHEST_PROTOCOL)
     p2 = pickle.loads(data)
     assert isinstance(p2, SimplePyDerived)
     assert p2.num == 202
     assert p2.stored_in_dict == 303
+    assert m.get_value_via_vtable_cpp(p2) == 100
 
 
 def test_roundtrip_simple_cpp_derived():
     p = m.make_SimpleCppDerivedAsBase()
     p.num = 404
+    assert m.get_value_via_vtable_cpp(p) == 10
     if not env.PYPY:
         # To ensure that this unit test is not accidentally invalidated.
         with pytest.raises(AttributeError):
@@ -77,3 +82,5 @@ def test_roundtrip_simple_cpp_derived():
     p2 = pickle.loads(data)
     assert isinstance(p2, m.SimpleBase)
     assert p2.num == 404
+    # ERROR: Fails due to object slicing.
+    assert m.get_value_via_vtable_cpp(p2) == 10