1Smart pointers
2##############
3
4std::unique_ptr
5===============
6
7Given a class ``Example`` with Python bindings, it's possible to return
8instances wrapped in C++11 unique pointers, like so
9
10.. code-block:: cpp
11
12    std::unique_ptr<Example> create_example() { return std::unique_ptr<Example>(new Example()); }
13
14.. code-block:: cpp
15
16    m.def("create_example", &create_example);
17
18In other words, there is nothing special that needs to be done. While returning
19unique pointers in this way is allowed, it is *illegal* to use them as function
20arguments. For instance, the following function signature cannot be processed
21by pybind11.
22
23.. code-block:: cpp
24
25    void do_something_with_example(std::unique_ptr<Example> ex) { ... }
26
27The above signature would imply that Python needs to give up ownership of an
28object that is passed to this function, which is generally not possible (for
29instance, the object might be referenced elsewhere).
30
31std::shared_ptr
32===============
33
34The binding generator for classes, :class:`class_`, can be passed a template
35type that denotes a special *holder* type that is used to manage references to
36the object.  If no such holder type template argument is given, the default for
37a type named ``Type`` is ``std::unique_ptr<Type>``, which means that the object
38is deallocated when Python's reference count goes to zero.
39
40It is possible to switch to other types of reference counting wrappers or smart
41pointers, which is useful in codebases that rely on them. For instance, the
42following snippet causes ``std::shared_ptr`` to be used instead.
43
44.. code-block:: cpp
45
46    py::class_<Example, std::shared_ptr<Example> /* <- holder type */> obj(m, "Example");
47
48Note that any particular class can only be associated with a single holder type.
49
50One potential stumbling block when using holder types is that they need to be
51applied consistently. Can you guess what's broken about the following binding
52code?
53
54.. code-block:: cpp
55
56    class Child { };
57
58    class Parent {
59    public:
60       Parent() : child(std::make_shared<Child>()) { }
61       Child *get_child() { return child.get(); }  /* Hint: ** DON'T DO THIS ** */
62    private:
63        std::shared_ptr<Child> child;
64    };
65
66    PYBIND11_MODULE(example, m) {
67        py::class_<Child, std::shared_ptr<Child>>(m, "Child");
68
69        py::class_<Parent, std::shared_ptr<Parent>>(m, "Parent")
70           .def(py::init<>())
71           .def("get_child", &Parent::get_child);
72    }
73
74The following Python code will cause undefined behavior (and likely a
75segmentation fault).
76
77.. code-block:: python
78
79   from example import Parent
80   print(Parent().get_child())
81
82The problem is that ``Parent::get_child()`` returns a pointer to an instance of
83``Child``, but the fact that this instance is already managed by
84``std::shared_ptr<...>`` is lost when passing raw pointers. In this case,
85pybind11 will create a second independent ``std::shared_ptr<...>`` that also
86claims ownership of the pointer. In the end, the object will be freed **twice**
87since these shared pointers have no way of knowing about each other.
88
89There are two ways to resolve this issue:
90
911. For types that are managed by a smart pointer class, never use raw pointers
92   in function arguments or return values. In other words: always consistently
93   wrap pointers into their designated holder types (such as
94   ``std::shared_ptr<...>``). In this case, the signature of ``get_child()``
95   should be modified as follows:
96
97.. code-block:: cpp
98
99    std::shared_ptr<Child> get_child() { return child; }
100
1012. Adjust the definition of ``Child`` by specifying
102   ``std::enable_shared_from_this<T>`` (see cppreference_ for details) as a
103   base class. This adds a small bit of information to ``Child`` that allows
104   pybind11 to realize that there is already an existing
105   ``std::shared_ptr<...>`` and communicate with it. In this case, the
106   declaration of ``Child`` should look as follows:
107
108.. _cppreference: http://en.cppreference.com/w/cpp/memory/enable_shared_from_this
109
110.. code-block:: cpp
111
112    class Child : public std::enable_shared_from_this<Child> { };
113
114.. _smart_pointers:
115
116Custom smart pointers
117=====================
118
119pybind11 supports ``std::unique_ptr`` and ``std::shared_ptr`` right out of the
120box. For any other custom smart pointer, transparent conversions can be enabled
121using a macro invocation similar to the following. It must be declared at the
122top namespace level before any binding code:
123
124.. code-block:: cpp
125
126    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>);
127
128The first argument of :func:`PYBIND11_DECLARE_HOLDER_TYPE` should be a
129placeholder name that is used as a template parameter of the second argument.
130Thus, feel free to use any identifier, but use it consistently on both sides;
131also, don't use the name of a type that already exists in your codebase.
132
133The macro also accepts a third optional boolean parameter that is set to false
134by default. Specify
135
136.. code-block:: cpp
137
138    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>, true);
139
140if ``SmartPtr<T>`` can always be initialized from a ``T*`` pointer without the
141risk of inconsistencies (such as multiple independent ``SmartPtr`` instances
142believing that they are the sole owner of the ``T*`` pointer). A common
143situation where ``true`` should be passed is when the ``T`` instances use
144*intrusive* reference counting.
145
146Please take a look at the :ref:`macro_notes` before using this feature.
147
148By default, pybind11 assumes that your custom smart pointer has a standard
149interface, i.e. provides a ``.get()`` member function to access the underlying
150raw pointer. If this is not the case, pybind11's ``holder_helper`` must be
151specialized:
152
153.. code-block:: cpp
154
155    // Always needed for custom holder types
156    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>);
157
158    // Only needed if the type's `.get()` goes by another name
159    namespace pybind11 { namespace detail {
160        template <typename T>
161        struct holder_helper<SmartPtr<T>> { // <-- specialization
162            static const T *get(const SmartPtr<T> &p) { return p.getPointer(); }
163        };
164    }}
165
166The above specialization informs pybind11 that the custom ``SmartPtr`` class
167provides ``.get()`` functionality via ``.getPointer()``.
168
169.. seealso::
170
171    The file :file:`tests/test_smart_ptr.cpp` contains a complete example
172    that demonstrates how to work with custom reference-counting holder types
173    in more detail.
174