always_construct_holder feature to support intrusively reference-counted types (#561)

* always_construct_holder feature to support intrusively reference-counted types

* added testcase

tests/test_smart_ptr.cpp

@@ -82,7 +82,11 @@ private:
 };
 
 /// Make pybind aware of the ref-counted wrapper type (s)
-PYBIND11_DECLARE_HOLDER_TYPE(T, ref<T>); // Required for custom holder type
+
+// ref<T> is a wrapper for 'Object' which uses intrusive reference counting
+// It is always possible to construct a ref<T> from an Object* pointer without
+// possible incosistencies, hence the 'true' argument at the end.
+PYBIND11_DECLARE_HOLDER_TYPE(T, ref<T>, true);
 PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>); // Not required any more for std::shared_ptr,
                                                      // but it should compile without error
 
@@ -125,6 +129,18 @@ test_initializer smart_ptr([](py::module &m) {
     py::class_<MyObject1, ref<MyObject1>>(m, "MyObject1", obj)
         .def(py::init<int>());
 
+    m.def("test_object1_refcounting",
+        []() -> bool {
+            ref<MyObject1> o = new MyObject1(0);
+            bool good = o->getRefCount() == 1;
+            py::object o2 = py::cast(o, py::return_value_policy::reference);
+            // always request (partial) ownership for objects with intrusive
+            // reference counting even when using the 'reference' RVP
+            good &= o->getRefCount() == 2;
+            return good;
+        }
+    );
+
     m.def("make_object_1", &make_object_1);
     m.def("make_object_2", &make_object_2);
     m.def("make_myobject1_1", &make_myobject1_1);