smart_ptrs.rst revision 11986:c12e4625ab56 
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_PLUGIN(example) {
67        py::module m("example");
68
69        py::class_<Child, std::shared_ptr<Child>>(m, "Child");
70
71        py::class_<Parent, std::shared_ptr<Parent>>(m, "Parent")
72           .def(py::init<>())
73           .def("get_child", &Parent::get_child);
74
75        return m.ptr();
76    }
77
78The following Python code will cause undefined behavior (and likely a
79segmentation fault).
80
81.. code-block:: python
82
83   from example import Parent
84   print(Parent().get_child())
85
86The problem is that ``Parent::get_child()`` returns a pointer to an instance of
87``Child``, but the fact that this instance is already managed by
88``std::shared_ptr<...>`` is lost when passing raw pointers. In this case,
89pybind11 will create a second independent ``std::shared_ptr<...>`` that also
90claims ownership of the pointer. In the end, the object will be freed **twice**
91since these shared pointers have no way of knowing about each other.
92
93There are two ways to resolve this issue:
94
951. For types that are managed by a smart pointer class, never use raw pointers
96   in function arguments or return values. In other words: always consistently
97   wrap pointers into their designated holder types (such as
98   ``std::shared_ptr<...>``). In this case, the signature of ``get_child()``
99   should be modified as follows:
100
101.. code-block:: cpp
102
103    std::shared_ptr<Child> get_child() { return child; }
104
1052. Adjust the definition of ``Child`` by specifying
106   ``std::enable_shared_from_this<T>`` (see cppreference_ for details) as a
107   base class. This adds a small bit of information to ``Child`` that allows
108   pybind11 to realize that there is already an existing
109   ``std::shared_ptr<...>`` and communicate with it. In this case, the
110   declaration of ``Child`` should look as follows:
111
112.. _cppreference: http://en.cppreference.com/w/cpp/memory/enable_shared_from_this
113
114.. code-block:: cpp
115
116    class Child : public std::enable_shared_from_this<Child> { };
117
118.. _smart_pointers:
119
120Custom smart pointers
121=====================
122
123pybind11 supports ``std::unique_ptr`` and ``std::shared_ptr`` right out of the
124box. For any other custom smart pointer, transparent conversions can be enabled
125using a macro invocation similar to the following. It must be declared at the
126level before any binding code:
127
128.. code-block:: cpp
129
130    PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>);
131
132The first argument of :func:`PYBIND11_DECLARE_HOLDER_TYPE` should be a
133placeholder name that is used as a template parameter of the second argument.
134Thus, feel free to use any identifier, but use it consistently on both sides;
135also, don't use the name of a type that already exists in your codebase.
136
137Please take a look at the :ref:`macro_notes` before using this feature.
138
139.. seealso::
140
141    The file :file:`tests/test_smart_ptr.cpp` contains a complete example
142    that demonstrates how to work with custom reference-counting holder types
143    in more detail.
144