improved shared pointer support (fixes #14)

docs/advanced.rst

@@ -457,6 +457,14 @@ be declared at the top level before any binding code:
     demonstrates how to work with custom reference-counting holder types in
     more detail.
 
+.. warning::
+
+   To ensure correct reference counting among Python and C++, the use of
+   ``std::shared_ptr<T>`` as a holder type requires that ``T`` inherits from
+   ``std::enable_shared_from_this<T>`` (see cppreference_ for details).
+
+.. _cppreference: http://en.cppreference.com/w/cpp/memory/enable_shared_from_this
+
 .. _custom_constructors:
 
 Custom constructors

example/example8.cpp

@@ -31,14 +31,36 @@ private:
     int value;
 };
 
+class MyObject2 : public std::enable_shared_from_this<MyObject2> {
+public:
+    MyObject2(int value) : value(value) {
+        std::cout << toString() << " constructor" << std::endl;
+    }
+
+    std::string toString() const {
+        return "MyObject2[" + std::to_string(value) + "]";
+    }
+
+    virtual ~MyObject2() {
+        std::cout << toString() << " destructor" << std::endl;
+    }
+
+private:
+    int value;
+};
+
 /// Make pybind aware of the ref-counted wrapper type
 PYBIND11_DECLARE_HOLDER_TYPE(T, ref<T>);
+PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>);
 
 Object *make_object_1() { return new MyObject(1); }
 ref<Object> make_object_2() { return new MyObject(2); }
 MyObject *make_myobject_4() { return new MyObject(4); }
 ref<MyObject> make_myobject_5() { return new MyObject(5); }
 
+MyObject2 *make_myobject2_1() { return new MyObject2(1); }
+std::shared_ptr<MyObject2> make_myobject2_2() { return std::make_shared<MyObject2>(2); }
+
 void print_object_1(const Object *obj) { std::cout << obj->toString() << std::endl; }
 void print_object_2(ref<Object> obj) { std::cout << obj->toString() << std::endl; }
 void print_object_3(const ref<Object> &obj) { std::cout << obj->toString() << std::endl; }
@@ -49,6 +71,11 @@ void print_myobject_2(ref<MyObject> obj) { std::cout << obj->toString() << std::
 void print_myobject_3(const ref<MyObject> &obj) { std::cout << obj->toString() << std::endl; }
 void print_myobject_4(const ref<MyObject> *obj) { std::cout << (*obj)->toString() << std::endl; }
 
+void print_myobject2_1(const MyObject2 *obj) { std::cout << obj->toString() << std::endl; }
+void print_myobject2_2(std::shared_ptr<MyObject2> obj) { std::cout << obj->toString() << std::endl; }
+void print_myobject2_3(const std::shared_ptr<MyObject2> &obj) { std::cout << obj->toString() << std::endl; }
+void print_myobject2_4(const std::shared_ptr<MyObject2> *obj) { std::cout << (*obj)->toString() << std::endl; }
+
 void init_ex8(py::module &m) {
     py::class_<Object, ref<Object>> obj(m, "Object");
     obj.def("getRefCount", &Object::getRefCount);
@@ -69,5 +96,14 @@ void init_ex8(py::module &m) {
     m.def("print_myobject_3", &print_myobject_3);
     m.def("print_myobject_4", &print_myobject_4);
 
+    py::class_<MyObject2, std::shared_ptr<MyObject2>>(m, "MyObject2")
+        .def(py::init<int>());
+    m.def("make_myobject2_1", &make_myobject2_1);
+    m.def("make_myobject2_2", &make_myobject2_2);
+    m.def("print_myobject2_1", &print_myobject2_1);
+    m.def("print_myobject2_2", &print_myobject2_2);
+    m.def("print_myobject2_3", &print_myobject2_3);
+    m.def("print_myobject2_4", &print_myobject2_4);
+
     py::implicitly_convertible<py::int_, MyObject>();
 }

example/example8.py

@@ -8,6 +8,8 @@ from example import make_object_1
 from example import make_object_2
 from example import make_myobject_4
 from example import make_myobject_5
+from example import make_myobject2_1
+from example import make_myobject2_2
 from example import print_object_1
 from example import print_object_2
 from example import print_object_3
@@ -16,6 +18,10 @@ from example import print_myobject_1
 from example import print_myobject_2
 from example import print_myobject_3
 from example import print_myobject_4
+from example import print_myobject2_1
+from example import print_myobject2_2
+from example import print_myobject2_3
+from example import print_myobject2_4
 
 for o in [make_object_1(), make_object_2(), MyObject(3)]:
     print("Reference count = %i" % o.getRefCount())
@@ -35,3 +41,11 @@ for o in [make_myobject_4(), make_myobject_5(), MyObject(6), 7]:
     print_myobject_2(o)
     print_myobject_3(o)
     print_myobject_4(o)
+
+
+for o in [make_myobject2_1(), make_myobject2_2()]:
+    print(o)
+    print_myobject2_1(o)
+    print_myobject2_2(o)
+    print_myobject2_3(o)
+    print_myobject2_4(o)

example/example8.ref

@@ -1,261 +1,275 @@
 MyObject[1] constructor
-Initialized ref from pointer 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
 MyObject[2] constructor
-Initialized ref from pointer 0x7ff7d9742150
-Initialized ref from pointer 0x7ff7d9742150
-Destructing ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Initialized ref from pointer 0x7ffdb2c167a0
+Destructing ref 0x7ffdb2c167a0
 MyObject[3] constructor
-Initialized ref from pointer 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
 Reference count = 1
 MyObject[1]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742750
+Initialized ref from ref 0x7ffdb2c16a50
 MyObject[1]
-Destructing ref 0x7ff7d9742750
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[1]
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[1]
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 Reference count = 1
 MyObject[2]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742150
+Initialized ref from ref 0x7ffdb2c167a0
 MyObject[2]
-Destructing ref 0x7ff7d9742150
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[2]
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[2]
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 Reference count = 1
 MyObject[3]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9741800
+Initialized ref from ref 0x7ffdb2c17420
 MyObject[3]
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c17420
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
 MyObject[3]
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
 MyObject[3]
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c167a0
 MyObject[2] destructor
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 MyObject[1] destructor
 MyObject[4] constructor
-Initialized ref from pointer 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
 MyObject[5] constructor
-Initialized ref from pointer 0x7ff7d9742150
-Initialized ref from pointer 0x7ff7d9742150
-Destructing ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Initialized ref from pointer 0x7ffdb2c167a0
+Destructing ref 0x7ffdb2c167a0
 MyObject[6] constructor
-Initialized ref from pointer 0x7ff7d9741cf0
-Destructing ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c161d0
+Destructing ref 0x7ffdb2c17420
 MyObject[3] destructor
-<example.MyObject object at 0x10297a360>
+<example.MyObject object at 0x10cf67600>
 MyObject[4]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742750
+Initialized ref from ref 0x7ffdb2c16a50
 MyObject[4]
-Destructing ref 0x7ff7d9742750
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[4]
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[4]
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 MyObject[4]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742750
+Initialized ref from ref 0x7ffdb2c16a50
 MyObject[4]
-Destructing ref 0x7ff7d9742750
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[4]
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742750
-Move-assigning ref 0x7ff7d9742750
+Initialized ref from pointer 0x7ffdb2c16a50
+Move-assigning ref 0x7ffdb2c16a50
 Destructing ref 0x0
 MyObject[4]
-Destructing ref 0x7ff7d9742750
-<example.MyObject object at 0x10297a870>
+Destructing ref 0x7ffdb2c16a50
+<example.MyObject object at 0x10cf7d210>
 MyObject[5]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742150
+Initialized ref from ref 0x7ffdb2c167a0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 MyObject[5]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9742150
+Initialized ref from ref 0x7ffdb2c167a0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9742150
-Move-assigning ref 0x7ff7d9742150
+Initialized ref from pointer 0x7ffdb2c167a0
+Move-assigning ref 0x7ffdb2c167a0
 Destructing ref 0x0
 MyObject[5]
-Destructing ref 0x7ff7d9742150
-<example.MyObject object at 0x10297a8d0>
+Destructing ref 0x7ffdb2c167a0
+<example.MyObject object at 0x10cf7d270>
 MyObject[6]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9741cf0
+Initialized ref from ref 0x7ffdb2c161d0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
+Destructing ref 0x7ffdb2c161d0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
 MyObject[6]
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9741cf0
+Initialized ref from ref 0x7ffdb2c161d0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
+Destructing ref 0x7ffdb2c161d0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
 Created empty ref
-Initialized ref from pointer 0x7ff7d9741cf0
-Move-assigning ref 0x7ff7d9741cf0
+Initialized ref from pointer 0x7ffdb2c161d0
+Move-assigning ref 0x7ffdb2c161d0
 Destructing ref 0x0
 MyObject[6]
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
 7
 MyObject[7] constructor
-Initialized ref from pointer 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
 MyObject[7]
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
 MyObject[7] destructor
 Created empty ref
 MyObject[7] constructor
-Initialized ref from pointer 0x7ff7d9741800
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
-Initialized ref from ref 0x7ff7d9741800
+Initialized ref from ref 0x7ffdb2c17420
 MyObject[7]
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c17420
 MyObject[7] destructor
 Created empty ref
 MyObject[7] constructor
-Initialized ref from pointer 0x7ff7d9741800
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
 MyObject[7]
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c17420
 MyObject[7] destructor
 Created empty ref
 MyObject[7] constructor
-Initialized ref from pointer 0x7ff7d9741800
-Initialized ref from pointer 0x7ff7d9741800
-Move-assigning ref 0x7ff7d9741800
+Initialized ref from pointer 0x7ffdb2c17420
+Initialized ref from pointer 0x7ffdb2c17420
+Move-assigning ref 0x7ffdb2c17420
 Destructing ref 0x0
 MyObject[7]
-Destructing ref 0x7ff7d9741800
-Destructing ref 0x7ff7d9741800
+Destructing ref 0x7ffdb2c17420
+Destructing ref 0x7ffdb2c17420
 MyObject[7] destructor
-Destructing ref 0x7ff7d9741cf0
+Destructing ref 0x7ffdb2c161d0
 MyObject[6] destructor
-Destructing ref 0x7ff7d9742150
+Destructing ref 0x7ffdb2c167a0
 MyObject[5] destructor
-Destructing ref 0x7ff7d9742750
+Destructing ref 0x7ffdb2c16a50
 MyObject[4] destructor
+MyObject2[1] constructor
+MyObject2[2] constructor
+<example.MyObject2 object at 0x10cf5c378>
+MyObject2[1]
+MyObject2[1]
+MyObject2[1]
+MyObject2[1]
+<example.MyObject2 object at 0x10cf5c340>
+MyObject2[2]
+MyObject2[2]
+MyObject2[2]
+MyObject2[2]
+MyObject2[1] destructor
+MyObject2[2] destructor

include/pybind11/cast.h

@@ -517,12 +517,25 @@ protected:
 template <typename type, typename holder_type> class type_caster_holder : public type_caster<type> {
 public:
     typedef type_caster<type> parent;
+
+    template <typename T = holder_type,
+              typename std::enable_if<std::is_same<std::shared_ptr<type>, T>::value, int>::type = 0>
+    bool load(PyObject *src, bool convert) {
+        if (!parent::load(src, convert))
+            return false;
+        holder = holder_type(((type *) parent::value)->shared_from_this());
+        return true;
+    }
+
+    template <typename T = holder_type,
+              typename std::enable_if<!std::is_same<std::shared_ptr<type>, T>::value, int>::type = 0>
     bool load(PyObject *src, bool convert) {
         if (!parent::load(src, convert))
             return false;
         holder = holder_type((type *) parent::value);
         return true;
     }
+
     explicit operator type*() { return this->value; }
     explicit operator type&() { return *(this->value); }
     explicit operator holder_type&() { return holder; }

include/pybind11/pybind11.h

@@ -798,11 +798,29 @@ public:
         return *this;
     }
 private:
+    template <typename T = holder_type,
+              typename std::enable_if<!std::is_same<std::shared_ptr<type>, T>::value, int>::type = 0>
     static void init_holder(PyObject *inst_) {
         instance_type *inst = (instance_type *) inst_;
         new (&inst->holder) holder_type(inst->value);
         inst->constructed = true;
     }
+
+    template <typename T = holder_type,
+              typename std::enable_if<std::is_same<std::shared_ptr<type>, T>::value, int>::type = 0>
+    static void init_holder(PyObject *inst_) {
+        instance_type *inst = (instance_type *) inst_;
+        try {
+            new (&inst->holder) holder_type(
+                inst->value->shared_from_this()
+            );
+        } catch (const std::bad_weak_ptr &) {
+            new (&inst->holder) holder_type(inst->value);
+        }
+        inst->constructed = true;
+    }
+
+
     static void dealloc(PyObject *inst_) {
         instance_type *inst = (instance_type *) inst_;
         if (inst->owned) {