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 Classes
 #######
 
 This section presents advanced binding code for classes and it is assumed
 that you are already familiar with the basics from :doc:`/classes`.
 
 .. _overriding_virtuals:
 
 Overriding virtual functions in Python
 ======================================
 
 Suppose that a C++ class or interface has a virtual function that we'd like
 to override from within Python (we'll focus on the class ``Animal``; ``Dog`` is
 given as a specific example of how one would do this with traditional C++
 code).
 
 .. code-block:: cpp
 
     class Animal {
     public:
         virtual ~Animal() { }
         virtual std::string go(int n_times) = 0;
     };
 
     class Dog : public Animal {
     public:
         std::string go(int n_times) override {
             std::string result;
             for (int i=0; i<n_times; ++i)
                 result += "woof! ";
             return result;
         }
     };
 
 Let's also suppose that we are given a plain function which calls the
 function ``go()`` on an arbitrary ``Animal`` instance.
 
 .. code-block:: cpp
 
     std::string call_go(Animal *animal) {
         return animal->go(3);
     }
 
 Normally, the binding code for these classes would look as follows:
 
 .. code-block:: cpp
 
     PYBIND11_MODULE(example, m) {
         py::class_<Animal>(m, "Animal")
             .def("go", &Animal::go);
 
         py::class_<Dog, Animal>(m, "Dog")
             .def(py::init<>());
 
         m.def("call_go", &call_go);
     }
 
 However, these bindings are impossible to extend: ``Animal`` is not
 constructible, and we clearly require some kind of "trampoline" that
 redirects virtual calls back to Python.
 
 Defining a new type of ``Animal`` from within Python is possible but requires a
 helper class that is defined as follows:
 
 .. code-block:: cpp
 
     class PyAnimal : public Animal {
     public:
         /* Inherit the constructors */
         using Animal::Animal;
 
         /* Trampoline (need one for each virtual function) */
         std::string go(int n_times) override {
             PYBIND11_OVERRIDE_PURE(
                 std::string, /* Return type */
                 Animal,      /* Parent class */
                 go,          /* Name of function in C++ (must match Python name) */
                 n_times      /* Argument(s) */
             );
         }
     };
 
 The macro :c:macro:`PYBIND11_OVERRIDE_PURE` should be used for pure virtual
 functions, and :c:macro:`PYBIND11_OVERRIDE` should be used for functions which have
 a default implementation.  There are also two alternate macros
 :c:macro:`PYBIND11_OVERRIDE_PURE_NAME` and :c:macro:`PYBIND11_OVERRIDE_NAME` which
 take a string-valued name argument between the *Parent class* and *Name of the
 function* slots, which defines the name of function in Python. This is required
 when the C++ and Python versions of the
 function have different names, e.g.  ``operator()`` vs ``__call__``.
 
 The binding code also needs a few minor adaptations (highlighted):
 
 .. code-block:: cpp
     :emphasize-lines: 2,3
 
     PYBIND11_MODULE(example, m) {
         py::class_<Animal, PyAnimal /* <--- trampoline*/>(m, "Animal")
             .def(py::init<>())
             .def("go", &Animal::go);
 
         py::class_<Dog, Animal>(m, "Dog")
             .def(py::init<>());
 
         m.def("call_go", &call_go);
     }
 
 Importantly, pybind11 is made aware of the trampoline helper class by
 specifying it as an extra template argument to :class:`class_`. (This can also
 be combined with other template arguments such as a custom holder type; the
 order of template types does not matter).  Following this, we are able to
 define a constructor as usual.
 
 Bindings should be made against the actual class, not the trampoline helper class.
 
 .. code-block:: cpp
     :emphasize-lines: 3
 
     py::class_<Animal, PyAnimal /* <--- trampoline*/>(m, "Animal");
         .def(py::init<>())
         .def("go", &PyAnimal::go); /* <--- THIS IS WRONG, use &Animal::go */
 
 Note, however, that the above is sufficient for allowing python classes to
 extend ``Animal``, but not ``Dog``: see :ref:`virtual_and_inheritance` for the
 necessary steps required to providing proper overriding support for inherited
 classes.
 
 The Python session below shows how to override ``Animal::go`` and invoke it via
 a virtual method call.
 
 .. code-block:: pycon
 
     >>> from example import *
     >>> d = Dog()
     >>> call_go(d)
     'woof! woof! woof! '
     >>> class Cat(Animal):
     ...     def go(self, n_times):
     ...         return "meow! " * n_times
     ...
     >>> c = Cat()
     >>> call_go(c)
     'meow! meow! meow! '
 
 If you are defining a custom constructor in a derived Python class, you *must*
 ensure that you explicitly call the bound C++ constructor using ``__init__``,
 *regardless* of whether it is a default constructor or not. Otherwise, the
 memory for the C++ portion of the instance will be left uninitialized, which
 will generally leave the C++ instance in an invalid state and cause undefined
 behavior if the C++ instance is subsequently used.
 
 .. versionchanged:: 2.6
    The default pybind11 metaclass will throw a ``TypeError`` when it detects
    that ``__init__`` was not called by a derived class.
 
 Here is an example:
 
 .. code-block:: python
 
     class Dachshund(Dog):
         def __init__(self, name):
             Dog.__init__(self)  # Without this, a TypeError is raised.
             self.name = name
 
         def bark(self):
             return "yap!"
 
 Note that a direct ``__init__`` constructor *should be called*, and ``super()``
 should not be used. For simple cases of linear inheritance, ``super()``
 may work, but once you begin mixing Python and C++ multiple inheritance,
 things will fall apart due to differences between Python's MRO and C++'s
 mechanisms.
 
 Please take a look at the :ref:`macro_notes` before using this feature.
 
 .. note::
 
     When the overridden type returns a reference or pointer to a type that
     pybind11 converts from Python (for example, numeric values, std::string,
     and other built-in value-converting types), there are some limitations to
     be aware of:
 
     - because in these cases there is no C++ variable to reference (the value
       is stored in the referenced Python variable), pybind11 provides one in
       the PYBIND11_OVERRIDE macros (when needed) with static storage duration.
       Note that this means that invoking the overridden method on *any*
       instance will change the referenced value stored in *all* instances of
       that type.
 
     - Attempts to modify a non-const reference will not have the desired
       effect: it will change only the static cache variable, but this change
       will not propagate to underlying Python instance, and the change will be
       replaced the next time the override is invoked.
 
 .. warning::
 
     The :c:macro:`PYBIND11_OVERRIDE` and accompanying macros used to be called
     ``PYBIND11_OVERLOAD`` up until pybind11 v2.5.0, and :func:`get_override`
     used to be called ``get_overload``. This naming was corrected and the older
     macro and function names may soon be deprecated, in order to reduce
     confusion with overloaded functions and methods and ``py::overload_cast``
     (see :ref:`classes`).
 
 .. seealso::
 
     The file :file:`tests/test_virtual_functions.cpp` contains a complete
     example that demonstrates how to override virtual functions using pybind11
     in more detail.
 
 .. _virtual_and_inheritance:
 
 Combining virtual functions and inheritance
 ===========================================
 
 When combining virtual methods with inheritance, you need to be sure to provide
 an override for each method for which you want to allow overrides from derived
 python classes.  For example, suppose we extend the above ``Animal``/``Dog``
 example as follows:
 
 .. code-block:: cpp
 
     class Animal {
     public:
         virtual std::string go(int n_times) = 0;
         virtual std::string name() { return "unknown"; }
     };
     class Dog : public Animal {
     public:
         std::string go(int n_times) override {
             std::string result;
             for (int i=0; i<n_times; ++i)
                 result += bark() + " ";
             return result;
         }
         virtual std::string bark() { return "woof!"; }
     };
 
 then the trampoline class for ``Animal`` must, as described in the previous
 section, override ``go()`` and ``name()``, but in order to allow python code to
 inherit properly from ``Dog``, we also need a trampoline class for ``Dog`` that
 overrides both the added ``bark()`` method *and* the ``go()`` and ``name()``
 methods inherited from ``Animal`` (even though ``Dog`` doesn't directly
 override the ``name()`` method):
 
 .. code-block:: cpp
 
     class PyAnimal : public Animal {
     public:
         using Animal::Animal; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, Animal, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, Animal, name, ); }
     };
     class PyDog : public Dog {
     public:
         using Dog::Dog; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE(std::string, Dog, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, Dog, name, ); }
         std::string bark() override { PYBIND11_OVERRIDE(std::string, Dog, bark, ); }
     };
 
 .. note::
 
     Note the trailing commas in the ``PYBIND11_OVERRIDE`` calls to ``name()``
     and ``bark()``. These are needed to portably implement a trampoline for a
     function that does not take any arguments. For functions that take
     a nonzero number of arguments, the trailing comma must be omitted.
 
 A registered class derived from a pybind11-registered class with virtual
 methods requires a similar trampoline class, *even if* it doesn't explicitly
 declare or override any virtual methods itself:
 
 .. code-block:: cpp
 
     class Husky : public Dog {};
     class PyHusky : public Husky {
     public:
         using Husky::Husky; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, Husky, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, Husky, name, ); }
         std::string bark() override { PYBIND11_OVERRIDE(std::string, Husky, bark, ); }
     };
 
 There is, however, a technique that can be used to avoid this duplication
 (which can be especially helpful for a base class with several virtual
 methods).  The technique involves using template trampoline classes, as
 follows:
 
 .. code-block:: cpp
 
     template <class AnimalBase = Animal> class PyAnimal : public AnimalBase {
     public:
         using AnimalBase::AnimalBase; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, AnimalBase, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, AnimalBase, name, ); }
     };
     template <class DogBase = Dog> class PyDog : public PyAnimal<DogBase> {
     public:
         using PyAnimal<DogBase>::PyAnimal; // Inherit constructors
         // Override PyAnimal's pure virtual go() with a non-pure one:
         std::string go(int n_times) override { PYBIND11_OVERRIDE(std::string, DogBase, go, n_times); }
         std::string bark() override { PYBIND11_OVERRIDE(std::string, DogBase, bark, ); }
     };
 
 This technique has the advantage of requiring just one trampoline method to be
 declared per virtual method and pure virtual method override.  It does,
 however, require the compiler to generate at least as many methods (and
 possibly more, if both pure virtual and overridden pure virtual methods are
 exposed, as above).
 
 The classes are then registered with pybind11 using:
 
 .. code-block:: cpp
 
     py::class_<Animal, PyAnimal<>> animal(m, "Animal");
     py::class_<Dog, Animal, PyDog<>> dog(m, "Dog");
     py::class_<Husky, Dog, PyDog<Husky>> husky(m, "Husky");
     // ... add animal, dog, husky definitions
 
 Note that ``Husky`` did not require a dedicated trampoline template class at
 all, since it neither declares any new virtual methods nor provides any pure
 virtual method implementations.
 
 With either the repeated-virtuals or templated trampoline methods in place, you
 can now create a python class that inherits from ``Dog``:
 
 .. code-block:: python
 
     class ShihTzu(Dog):
         def bark(self):
             return "yip!"
 
 .. seealso::
 
     See the file :file:`tests/test_virtual_functions.cpp` for complete examples
     using both the duplication and templated trampoline approaches.
 
 .. _extended_aliases:
 
 Extended trampoline class functionality
 =======================================
 
 .. _extended_class_functionality_forced_trampoline:
 
 Forced trampoline class initialisation
 --------------------------------------
 The trampoline classes described in the previous sections are, by default, only
 initialized when needed.  More specifically, they are initialized when a python
 class actually inherits from a registered type (instead of merely creating an
 instance of the registered type), or when a registered constructor is only
 valid for the trampoline class but not the registered class.  This is primarily
 for performance reasons: when the trampoline class is not needed for anything
 except virtual method dispatching, not initializing the trampoline class
 improves performance by avoiding needing to do a run-time check to see if the
 inheriting python instance has an overridden method.
 
 Sometimes, however, it is useful to always initialize a trampoline class as an
 intermediate class that does more than just handle virtual method dispatching.
 For example, such a class might perform extra class initialization, extra
 destruction operations, and might define new members and methods to enable a
 more python-like interface to a class.
 
 In order to tell pybind11 that it should *always* initialize the trampoline
 class when creating new instances of a type, the class constructors should be
 declared using ``py::init_alias<Args, ...>()`` instead of the usual
 ``py::init<Args, ...>()``.  This forces construction via the trampoline class,
 ensuring member initialization and (eventual) destruction.
 
 .. seealso::
 
     See the file :file:`tests/test_virtual_functions.cpp` for complete examples
     showing both normal and forced trampoline instantiation.
 
 Different method signatures
 ---------------------------
 The macro's introduced in :ref:`overriding_virtuals` cover most of the standard
 use cases when exposing C++ classes to Python. Sometimes it is hard or unwieldy
 to create a direct one-on-one mapping between the arguments and method return
 type.
 
 An example would be when the C++ signature contains output arguments using
 references (See also :ref:`faq_reference_arguments`). Another way of solving
 this is to use the method body of the trampoline class to do conversions to the
 input and return of the Python method.
 
 The main building block to do so is the :func:`get_override`, this function
 allows retrieving a method implemented in Python from within the trampoline's
 methods. Consider for example a C++ method which has the signature
 ``bool myMethod(int32_t& value)``, where the return indicates whether
 something should be done with the ``value``. This can be made convenient on the
 Python side by allowing the Python function to return ``None`` or an ``int``:
 
 .. code-block:: cpp
 
     bool MyClass::myMethod(int32_t& value)
     {
         pybind11::gil_scoped_acquire gil;  // Acquire the GIL while in this scope.
         // Try to look up the overridden method on the Python side.
         pybind11::function override = pybind11::get_override(this, "myMethod");
         if (override) {  // method is found
             auto obj = override(value);  // Call the Python function.
             if (py::isinstance<py::int_>(obj)) {  // check if it returned a Python integer type
                 value = obj.cast<int32_t>();  // Cast it and assign it to the value.
                 return true;  // Return true; value should be used.
             } else {
                 return false;  // Python returned none, return false.
             }
         }
         return false;  // Alternatively return MyClass::myMethod(value);
     }
 
 
 .. _custom_constructors:
 
 Custom constructors
 ===================
 
 The syntax for binding constructors was previously introduced, but it only
 works when a constructor of the appropriate arguments actually exists on the
 C++ side.  To extend this to more general cases, pybind11 makes it possible
 to bind factory functions as constructors. For example, suppose you have a
 class like this:
 
 .. code-block:: cpp
 
     class Example {
     private:
         Example(int); // private constructor
     public:
         // Factory function:
         static Example create(int a) { return Example(a); }
     };
 
     py::class_<Example>(m, "Example")
         .def(py::init(&Example::create));
 
 While it is possible to create a straightforward binding of the static
 ``create`` method, it may sometimes be preferable to expose it as a constructor
 on the Python side. This can be accomplished by calling ``.def(py::init(...))``
 with the function reference returning the new instance passed as an argument.
 It is also possible to use this approach to bind a function returning a new
 instance by raw pointer or by the holder (e.g. ``std::unique_ptr``).
 
 The following example shows the different approaches:
 
 .. code-block:: cpp
 
     class Example {
     private:
         Example(int); // private constructor
     public:
         // Factory function - returned by value:
         static Example create(int a) { return Example(a); }
 
         // These constructors are publicly callable:
         Example(double);
         Example(int, int);
         Example(std::string);
     };
 
     py::class_<Example>(m, "Example")
         // Bind the factory function as a constructor:
         .def(py::init(&Example::create))
         // Bind a lambda function returning a pointer wrapped in a holder:
         .def(py::init([](std::string arg) {
             return std::unique_ptr<Example>(new Example(arg));
         }))
         // Return a raw pointer:
         .def(py::init([](int a, int b) { return new Example(a, b); }))
         // You can mix the above with regular C++ constructor bindings as well:
         .def(py::init<double>())
         ;
 
 When the constructor is invoked from Python, pybind11 will call the factory
 function and store the resulting C++ instance in the Python instance.
 
 When combining factory functions constructors with :ref:`virtual function
 trampolines <overriding_virtuals>` there are two approaches.  The first is to
 add a constructor to the alias class that takes a base value by
 rvalue-reference.  If such a constructor is available, it will be used to
 construct an alias instance from the value returned by the factory function.
 The second option is to provide two factory functions to ``py::init()``: the
 first will be invoked when no alias class is required (i.e. when the class is
 being used but not inherited from in Python), and the second will be invoked
 when an alias is required.
 
 You can also specify a single factory function that always returns an alias
 instance: this will result in behaviour similar to ``py::init_alias<...>()``,
 as described in the :ref:`extended trampoline class documentation
 <extended_aliases>`.
 
 The following example shows the different factory approaches for a class with
 an alias:
 
 .. code-block:: cpp
 
     #include <pybind11/factory.h>
     class Example {
     public:
         // ...
         virtual ~Example() = default;
     };
     class PyExample : public Example {
     public:
         using Example::Example;
         PyExample(Example &&base) : Example(std::move(base)) {}
     };
     py::class_<Example, PyExample>(m, "Example")
         // Returns an Example pointer.  If a PyExample is needed, the Example
         // instance will be moved via the extra constructor in PyExample, above.
         .def(py::init([]() { return new Example(); }))
         // Two callbacks:
         .def(py::init([]() { return new Example(); } /* no alias needed */,
                       []() { return new PyExample(); } /* alias needed */))
         // *Always* returns an alias instance (like py::init_alias<>())
         .def(py::init([]() { return new PyExample(); }))
         ;
 
 Brace initialization
 --------------------
 
 ``pybind11::init<>`` internally uses C++11 brace initialization to call the
 constructor of the target class. This means that it can be used to bind
 *implicit* constructors as well:
 
 .. code-block:: cpp
 
     struct Aggregate {
         int a;
         std::string b;
     };
 
     py::class_<Aggregate>(m, "Aggregate")
         .def(py::init<int, const std::string &>());
 
 .. note::
 
     Note that brace initialization preferentially invokes constructor overloads
     taking a ``std::initializer_list``. In the rare event that this causes an
     issue, you can work around it by using ``py::init(...)`` with a lambda
     function that constructs the new object as desired.
 
 .. _classes_with_non_public_destructors:
 
 Non-public destructors
 ======================
 
 If a class has a private or protected destructor (as might e.g. be the case in
 a singleton pattern), a compile error will occur when creating bindings via
 pybind11. The underlying issue is that the ``std::unique_ptr`` holder type that
 is responsible for managing the lifetime of instances will reference the
 destructor even if no deallocations ever take place. In order to expose classes
 with private or protected destructors, it is possible to override the holder
 type via a holder type argument to ``class_``. Pybind11 provides a helper class
 ``py::nodelete`` that disables any destructor invocations. In this case, it is
 crucial that instances are deallocated on the C++ side to avoid memory leaks.
 
 .. code-block:: cpp
 
     /* ... definition ... */
 
     class MyClass {
     private:
         ~MyClass() { }
     };
 
     /* ... binding code ... */
 
     py::class_<MyClass, std::unique_ptr<MyClass, py::nodelete>>(m, "MyClass")
         .def(py::init<>())
 
 .. _destructors_that_call_python:
 
 Destructors that call Python
 ============================
 
 If a Python function is invoked from a C++ destructor, an exception may be thrown
 of type :class:`error_already_set`. If this error is thrown out of a class destructor,
 ``std::terminate()`` will be called, terminating the process. Class destructors
 must catch all exceptions of type :class:`error_already_set` to discard the Python
 exception using :func:`error_already_set::discard_as_unraisable`.
 
 Every Python function should be treated as *possibly throwing*. When a Python generator
 stops yielding items, Python will throw a ``StopIteration`` exception, which can pass
 though C++ destructors if the generator's stack frame holds the last reference to C++
 objects.
 
 For more information, see :ref:`the documentation on exceptions <unraisable_exceptions>`.
 
 .. code-block:: cpp
 
     class MyClass {
     public:
         ~MyClass() {
             try {
                 py::print("Even printing is dangerous in a destructor");
                 py::exec("raise ValueError('This is an unraisable exception')");
             } catch (py::error_already_set &e) {
                 // error_context should be information about where/why the occurred,
                 // e.g. use __func__ to get the name of the current function
                 e.discard_as_unraisable(__func__);
             }
         }
     };
 
 .. note::
 
     pybind11 does not support C++ destructors marked ``noexcept(false)``.
 
 .. versionadded:: 2.6
 
 .. _implicit_conversions:
 
 Implicit conversions
 ====================
 
 Suppose that instances of two types ``A`` and ``B`` are used in a project, and
 that an ``A`` can easily be converted into an instance of type ``B`` (examples of this
 could be a fixed and an arbitrary precision number type).
 
 .. code-block:: cpp
 
     py::class_<A>(m, "A")
         /// ... members ...
 
     py::class_<B>(m, "B")
         .def(py::init<A>())
         /// ... members ...
 
     m.def("func",
         [](const B &) { /* .... */ }
     );
 
 To invoke the function ``func`` using a variable ``a`` containing an ``A``
 instance, we'd have to write ``func(B(a))`` in Python. On the other hand, C++
 will automatically apply an implicit type conversion, which makes it possible
 to directly write ``func(a)``.
 
 In this situation (i.e. where ``B`` has a constructor that converts from
 ``A``), the following statement enables similar implicit conversions on the
 Python side:
 
 .. code-block:: cpp
 
     py::implicitly_convertible<A, B>();
 
 .. note::
 
     Implicit conversions from ``A`` to ``B`` only work when ``B`` is a custom
     data type that is exposed to Python via pybind11.
 
     To prevent runaway recursion, implicit conversions are non-reentrant: an
     implicit conversion invoked as part of another implicit conversion of the
     same type (i.e. from ``A`` to ``B``) will fail.
 
 .. _static_properties:
 
 Static properties
 =================
 
 The section on :ref:`properties` discussed the creation of instance properties
 that are implemented in terms of C++ getters and setters.
 
 Static properties can also be created in a similar way to expose getters and
 setters of static class attributes. Note that the implicit ``self`` argument
 also exists in this case and is used to pass the Python ``type`` subclass
 instance. This parameter will often not be needed by the C++ side, and the
 following example illustrates how to instantiate a lambda getter function
 that ignores it:
 
 .. code-block:: cpp
 
     py::class_<Foo>(m, "Foo")
         .def_property_readonly_static("foo", [](py::object /* self */) { return Foo(); });
 
 Operator overloading
 ====================
 
 Suppose that we're given the following ``Vector2`` class with a vector addition
 and scalar multiplication operation, all implemented using overloaded operators
 in C++.
 
 .. code-block:: cpp
 
     class Vector2 {
     public:
         Vector2(float x, float y) : x(x), y(y) { }
 
         Vector2 operator+(const Vector2 &v) const { return Vector2(x + v.x, y + v.y); }
         Vector2 operator*(float value) const { return Vector2(x * value, y * value); }
         Vector2& operator+=(const Vector2 &v) { x += v.x; y += v.y; return *this; }
         Vector2& operator*=(float v) { x *= v; y *= v; return *this; }
 
         friend Vector2 operator*(float f, const Vector2 &v) {
             return Vector2(f * v.x, f * v.y);
         }
 
         std::string toString() const {
             return "[" + std::to_string(x) + ", " + std::to_string(y) + "]";
         }
     private:
         float x, y;
     };
 
 The following snippet shows how the above operators can be conveniently exposed
 to Python.
 
 .. code-block:: cpp
 
     #include <pybind11/operators.h>
 
     PYBIND11_MODULE(example, m) {
         py::class_<Vector2>(m, "Vector2")
             .def(py::init<float, float>())
             .def(py::self + py::self)
             .def(py::self += py::self)
             .def(py::self *= float())
             .def(float() * py::self)
             .def(py::self * float())
             .def(-py::self)
             .def("__repr__", &Vector2::toString);
     }
 
 Note that a line like
 
 .. code-block:: cpp
 
             .def(py::self * float())
 
 is really just short hand notation for
 
 .. code-block:: cpp
 
     .def("__mul__", [](const Vector2 &a, float b) {
         return a * b;
     }, py::is_operator())
 
 This can be useful for exposing additional operators that don't exist on the
 C++ side, or to perform other types of customization. The ``py::is_operator``
 flag marker is needed to inform pybind11 that this is an operator, which
 returns ``NotImplemented`` when invoked with incompatible arguments rather than
 throwing a type error.
 
 .. note::
 
     To use the more convenient ``py::self`` notation, the additional
     header file :file:`pybind11/operators.h` must be included.
 
 .. seealso::
 
     The file :file:`tests/test_operator_overloading.cpp` contains a
     complete example that demonstrates how to work with overloaded operators in
     more detail.
 
 .. _pickling:
 
 Pickling support
 ================
 
 Python's ``pickle`` module provides a powerful facility to serialize and
 de-serialize a Python object graph into a binary data stream. To pickle and
 unpickle C++ classes using pybind11, a ``py::pickle()`` definition must be
 provided. Suppose the class in question has the following signature:
 
 .. code-block:: cpp
 
     class Pickleable {
     public:
         Pickleable(const std::string &value) : m_value(value) { }
         const std::string &value() const { return m_value; }
 
         void setExtra(int extra) { m_extra = extra; }
         int extra() const { return m_extra; }
     private:
         std::string m_value;
         int m_extra = 0;
     };
 
 Pickling support in Python is enabled by defining the ``__setstate__`` and
 ``__getstate__`` methods [#f3]_. For pybind11 classes, use ``py::pickle()``
 to bind these two functions:
 
 .. code-block:: cpp
 
     py::class_<Pickleable>(m, "Pickleable")
         .def(py::init<std::string>())
         .def("value", &Pickleable::value)
         .def("extra", &Pickleable::extra)
         .def("setExtra", &Pickleable::setExtra)
         .def(py::pickle(
             [](const Pickleable &p) { // __getstate__
                 /* Return a tuple that fully encodes the state of the object */
                 return py::make_tuple(p.value(), p.extra());
             },
             [](py::tuple t) { // __setstate__
                 if (t.size() != 2)
                     throw std::runtime_error("Invalid state!");
 
                 /* Create a new C++ instance */
                 Pickleable p(t[0].cast<std::string>());
 
                 /* Assign any additional state */
                 p.setExtra(t[1].cast<int>());
 
                 return p;
             }
         ));
 
 The ``__setstate__`` part of the ``py::pickle()`` definition follows the same
 rules as the single-argument version of ``py::init()``. The return type can be
 a value, pointer or holder type. See :ref:`custom_constructors` for details.
 
 An instance can now be pickled as follows:
 
 .. code-block:: python
 
     import pickle
 
     p = Pickleable("test_value")
     p.setExtra(15)
     data = pickle.dumps(p)
 
 
 .. note::
     If given, the second argument to ``dumps`` must be 2 or larger - 0 and 1 are
     not supported. Newer versions are also fine; for instance, specify ``-1`` to
     always use the latest available version. Beware: failure to follow these
     instructions will cause important pybind11 memory allocation routines to be
     skipped during unpickling, which will likely lead to memory corruption
     and/or segmentation faults. Python defaults to version 3 (Python 3-3.7) and
     version 4 for Python 3.8+.
 
 .. seealso::
 
     The file :file:`tests/test_pickling.cpp` contains a complete example
     that demonstrates how to pickle and unpickle types using pybind11 in more
     detail.
 
 .. [#f3] http://docs.python.org/3/library/pickle.html#pickling-class-instances
 
 Deepcopy support
 ================
 
 Python normally uses references in assignments. Sometimes a real copy is needed
 to prevent changing all copies. The ``copy`` module [#f5]_ provides these
 capabilities.
 
 A class with pickle support is automatically also (deep)copy
 compatible. However, performance can be improved by adding custom
 ``__copy__`` and ``__deepcopy__`` methods.
 
 For simple classes (deep)copy can be enabled by using the copy constructor,
 which should look as follows:
 
 .. code-block:: cpp
 
     py::class_<Copyable>(m, "Copyable")
         .def("__copy__",  [](const Copyable &self) {
             return Copyable(self);
         })
         .def("__deepcopy__", [](const Copyable &self, py::dict) {
             return Copyable(self);
         }, "memo"_a);
 
 .. note::
 
     Dynamic attributes will not be copied in this example.
 
 .. [#f5] https://docs.python.org/3/library/copy.html
 
 Multiple Inheritance
 ====================
 
 pybind11 can create bindings for types that derive from multiple base types
 (aka. *multiple inheritance*). To do so, specify all bases in the template
 arguments of the ``class_`` declaration:
 
 .. code-block:: cpp
 
     py::class_<MyType, BaseType1, BaseType2, BaseType3>(m, "MyType")
        ...
 
 The base types can be specified in arbitrary order, and they can even be
 interspersed with alias types and holder types (discussed earlier in this
 document)---pybind11 will automatically find out which is which. The only
 requirement is that the first template argument is the type to be declared.
 
 It is also permitted to inherit multiply from exported C++ classes in Python,
 as well as inheriting from multiple Python and/or pybind11-exported classes.
 
 There is one caveat regarding the implementation of this feature:
 
 When only one base type is specified for a C++ type that actually has multiple
 bases, pybind11 will assume that it does not participate in multiple
 inheritance, which can lead to undefined behavior. In such cases, add the tag
 ``multiple_inheritance`` to the class constructor:
 
 .. code-block:: cpp
 
     py::class_<MyType, BaseType2>(m, "MyType", py::multiple_inheritance());
 
 The tag is redundant and does not need to be specified when multiple base types
 are listed.
 
 .. _module_local:
 
 Module-local class bindings
 ===========================
 
 When creating a binding for a class, pybind11 by default makes that binding
 "global" across modules.  What this means is that a type defined in one module
 can be returned from any module resulting in the same Python type.  For
 example, this allows the following:
 
 .. code-block:: cpp
 
     // In the module1.cpp binding code for module1:
     py::class_<Pet>(m, "Pet")
         .def(py::init<std::string>())
         .def_readonly("name", &Pet::name);
 
 .. code-block:: cpp
 
     // In the module2.cpp binding code for module2:
     m.def("create_pet", [](std::string name) { return new Pet(name); });
 
 .. code-block:: pycon
 
     >>> from module1 import Pet
     >>> from module2 import create_pet
     >>> pet1 = Pet("Kitty")
     >>> pet2 = create_pet("Doggy")
     >>> pet2.name()
     'Doggy'
 
 When writing binding code for a library, this is usually desirable: this
 allows, for example, splitting up a complex library into multiple Python
 modules.
 
 In some cases, however, this can cause conflicts.  For example, suppose two
 unrelated modules make use of an external C++ library and each provide custom
 bindings for one of that library's classes.  This will result in an error when
 a Python program attempts to import both modules (directly or indirectly)
 because of conflicting definitions on the external type:
 
 .. code-block:: cpp
 
     // dogs.cpp
 
     // Binding for external library class:
     py::class<pets::Pet>(m, "Pet")
         .def("name", &pets::Pet::name);
 
     // Binding for local extension class:
     py::class<Dog, pets::Pet>(m, "Dog")
         .def(py::init<std::string>());
 
 .. code-block:: cpp
 
     // cats.cpp, in a completely separate project from the above dogs.cpp.
 
     // Binding for external library class:
     py::class<pets::Pet>(m, "Pet")
         .def("get_name", &pets::Pet::name);
 
     // Binding for local extending class:
     py::class<Cat, pets::Pet>(m, "Cat")
         .def(py::init<std::string>());
 
 .. code-block:: pycon
 
     >>> import cats
     >>> import dogs
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     ImportError: generic_type: type "Pet" is already registered!
 
 To get around this, you can tell pybind11 to keep the external class binding
 localized to the module by passing the ``py::module_local()`` attribute into
 the ``py::class_`` constructor:
 
 .. code-block:: cpp
 
     // Pet binding in dogs.cpp:
     py::class<pets::Pet>(m, "Pet", py::module_local())
         .def("name", &pets::Pet::name);
 
 .. code-block:: cpp
 
     // Pet binding in cats.cpp:
     py::class<pets::Pet>(m, "Pet", py::module_local())
         .def("get_name", &pets::Pet::name);
 
 This makes the Python-side ``dogs.Pet`` and ``cats.Pet`` into distinct classes,
 avoiding the conflict and allowing both modules to be loaded.  C++ code in the
 ``dogs`` module that casts or returns a ``Pet`` instance will result in a
 ``dogs.Pet`` Python instance, while C++ code in the ``cats`` module will result
 in a ``cats.Pet`` Python instance.
 
 This does come with two caveats, however: First, external modules cannot return
 or cast a ``Pet`` instance to Python (unless they also provide their own local
 bindings).  Second, from the Python point of view they are two distinct classes.
 
 Note that the locality only applies in the C++ -> Python direction.  When
 passing such a ``py::module_local`` type into a C++ function, the module-local
 classes are still considered.  This means that if the following function is
 added to any module (including but not limited to the ``cats`` and ``dogs``
 modules above) it will be callable with either a ``dogs.Pet`` or ``cats.Pet``
 argument:
 
 .. code-block:: cpp
 
     m.def("pet_name", [](const pets::Pet &pet) { return pet.name(); });
 
 For example, suppose the above function is added to each of ``cats.cpp``,
 ``dogs.cpp`` and ``frogs.cpp`` (where ``frogs.cpp`` is some other module that
 does *not* bind ``Pets`` at all).
 
 .. code-block:: pycon
 
     >>> import cats, dogs, frogs  # No error because of the added py::module_local()
     >>> mycat, mydog = cats.Cat("Fluffy"), dogs.Dog("Rover")
     >>> (cats.pet_name(mycat), dogs.pet_name(mydog))
     ('Fluffy', 'Rover')
     >>> (cats.pet_name(mydog), dogs.pet_name(mycat), frogs.pet_name(mycat))
     ('Rover', 'Fluffy', 'Fluffy')
 
 It is possible to use ``py::module_local()`` registrations in one module even
 if another module registers the same type globally: within the module with the
 module-local definition, all C++ instances will be cast to the associated bound
 Python type.  In other modules any such values are converted to the global
 Python type created elsewhere.
 
 .. note::
 
     STL bindings (as provided via the optional :file:`pybind11/stl_bind.h`
     header) apply ``py::module_local`` by default when the bound type might
     conflict with other modules; see :ref:`stl_bind` for details.
 
 .. note::
 
     The localization of the bound types is actually tied to the shared object
     or binary generated by the compiler/linker.  For typical modules created
     with ``PYBIND11_MODULE()``, this distinction is not significant.  It is
     possible, however, when :ref:`embedding` to embed multiple modules in the
     same binary (see :ref:`embedding_modules`).  In such a case, the
     localization will apply across all embedded modules within the same binary.
 
 .. seealso::
 
     The file :file:`tests/test_local_bindings.cpp` contains additional examples
     that demonstrate how ``py::module_local()`` works.
 
 Binding protected member functions
 ==================================
 
 It's normally not possible to expose ``protected`` member functions to Python:
 
 .. code-block:: cpp
 
     class A {
     protected:
         int foo() const { return 42; }
     };
 
     py::class_<A>(m, "A")
         .def("foo", &A::foo); // error: 'foo' is a protected member of 'A'
 
 On one hand, this is good because non-``public`` members aren't meant to be
 accessed from the outside. But we may want to make use of ``protected``
 functions in derived Python classes.
 
 The following pattern makes this possible:
 
 .. code-block:: cpp
 
     class A {
     protected:
         int foo() const { return 42; }
     };
 
     class Publicist : public A { // helper type for exposing protected functions
     public:
         using A::foo; // inherited with different access modifier
     };
 
     py::class_<A>(m, "A") // bind the primary class
         .def("foo", &Publicist::foo); // expose protected methods via the publicist
 
 This works because ``&Publicist::foo`` is exactly the same function as
 ``&A::foo`` (same signature and address), just with a different access
 modifier. The only purpose of the ``Publicist`` helper class is to make
 the function name ``public``.
 
 If the intent is to expose ``protected`` ``virtual`` functions which can be
 overridden in Python, the publicist pattern can be combined with the previously
 described trampoline:
 
 .. code-block:: cpp
 
     class A {
     public:
         virtual ~A() = default;
 
     protected:
         virtual int foo() const { return 42; }
     };
 
     class Trampoline : public A {
     public:
         int foo() const override { PYBIND11_OVERRIDE(int, A, foo, ); }
     };
 
     class Publicist : public A {
     public:
         using A::foo;
     };
 
     py::class_<A, Trampoline>(m, "A") // <-- `Trampoline` here
         .def("foo", &Publicist::foo); // <-- `Publicist` here, not `Trampoline`!
 
 Binding final classes
 =====================
 
 Some classes may not be appropriate to inherit from. In C++11, classes can
 use the ``final`` specifier to ensure that a class cannot be inherited from.
 The ``py::is_final`` attribute can be used to ensure that Python classes
 cannot inherit from a specified type. The underlying C++ type does not need
 to be declared final.
 
 .. code-block:: cpp
 
     class IsFinal final {};
 
     py::class_<IsFinal>(m, "IsFinal", py::is_final());
 
 When you try to inherit from such a class in Python, you will now get this
 error:
 
 .. code-block:: pycon
 
     >>> class PyFinalChild(IsFinal):
     ...     pass
     ...
     TypeError: type 'IsFinal' is not an acceptable base type
 
 .. note:: This attribute is currently ignored on PyPy
 
 .. versionadded:: 2.6
 
 Binding classes with template parameters
 ========================================
 
 pybind11 can also wrap classes that have template parameters. Consider these classes:
 
 .. code-block:: cpp
 
     struct Cat {};
     struct Dog {};
 
     template <typename PetType>
     struct Cage {
         Cage(PetType& pet);
         PetType& get();
     };
 
 C++ templates may only be instantiated at compile time, so pybind11 can only
 wrap instantiated templated classes. You cannot wrap a non-instantiated template:
 
 .. code-block:: cpp
 
     // BROKEN (this will not compile)
     py::class_<Cage>(m, "Cage");
         .def("get", &Cage::get);
 
 You must explicitly specify each template/type combination that you want to
 wrap separately.
 
 .. code-block:: cpp
 
     // ok
     py::class_<Cage<Cat>>(m, "CatCage")
         .def("get", &Cage<Cat>::get);
 
     // ok
     py::class_<Cage<Dog>>(m, "DogCage")
         .def("get", &Cage<Dog>::get);
 
 If your class methods have template parameters you can wrap those as well,
 but once again each instantiation must be explicitly specified:
 
 .. code-block:: cpp
 
     typename <typename T>
     struct MyClass {
         template <typename V>
         T fn(V v);
     };
 
     py::class<MyClass<int>>(m, "MyClassT")
         .def("fn", &MyClass<int>::fn<std::string>);
 
 Custom automatic downcasters
 ============================
 
 As explained in :ref:`inheritance`, pybind11 comes with built-in
 understanding of the dynamic type of polymorphic objects in C++; that
 is, returning a Pet to Python produces a Python object that knows it's
 wrapping a Dog, if Pet has virtual methods and pybind11 knows about
 Dog and this Pet is in fact a Dog. Sometimes, you might want to
 provide this automatic downcasting behavior when creating bindings for
 a class hierarchy that does not use standard C++ polymorphism, such as
 LLVM [#f4]_. As long as there's some way to determine at runtime
 whether a downcast is safe, you can proceed by specializing the
 ``pybind11::polymorphic_type_hook`` template:
 
 .. code-block:: cpp
 
     enum class PetKind { Cat, Dog, Zebra };
     struct Pet {   // Not polymorphic: has no virtual methods
         const PetKind kind;
         int age = 0;
       protected:
         Pet(PetKind _kind) : kind(_kind) {}
     };
     struct Dog : Pet {
         Dog() : Pet(PetKind::Dog) {}
         std::string sound = "woof!";
         std::string bark() const { return sound; }
     };
 
     namespace PYBIND11_NAMESPACE {
         template<> struct polymorphic_type_hook<Pet> {
             static const void *get(const Pet *src, const std::type_info*& type) {
                 // note that src may be nullptr
                 if (src && src->kind == PetKind::Dog) {
                     type = &typeid(Dog);
                     return static_cast<const Dog*>(src);
                 }
                 return src;
             }
         };
     } // namespace PYBIND11_NAMESPACE
 
 When pybind11 wants to convert a C++ pointer of type ``Base*`` to a
 Python object, it calls ``polymorphic_type_hook<Base>::get()`` to
 determine if a downcast is possible. The ``get()`` function should use
 whatever runtime information is available to determine if its ``src``
 parameter is in fact an instance of some class ``Derived`` that
 inherits from ``Base``. If it finds such a ``Derived``, it sets ``type
 = &typeid(Derived)`` and returns a pointer to the ``Derived`` object
 that contains ``src``. Otherwise, it just returns ``src``, leaving
 ``type`` at its default value of nullptr. If you set ``type`` to a
 type that pybind11 doesn't know about, no downcasting will occur, and
 the original ``src`` pointer will be used with its static type
 ``Base*``.
 
 It is critical that the returned pointer and ``type`` argument of
 ``get()`` agree with each other: if ``type`` is set to something
 non-null, the returned pointer must point to the start of an object
 whose type is ``type``. If the hierarchy being exposed uses only
 single inheritance, a simple ``return src;`` will achieve this just
 fine, but in the general case, you must cast ``src`` to the
 appropriate derived-class pointer (e.g. using
 ``static_cast<Derived>(src)``) before allowing it to be returned as a
 ``void*``.
 
 .. [#f4] https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html
 
 .. note::
 
     pybind11's standard support for downcasting objects whose types
     have virtual methods is implemented using
     ``polymorphic_type_hook`` too, using the standard C++ ability to
     determine the most-derived type of a polymorphic object using
     ``typeid()`` and to cast a base pointer to that most-derived type
     (even if you don't know what it is) using ``dynamic_cast<void*>``.
 
 .. seealso::
 
     The file :file:`tests/test_tagbased_polymorphic.cpp` contains a
     more complete example, including a demonstration of how to provide
     automatic downcasting for an entire class hierarchy without
     writing one get() function for each class.
 
 Accessing the type object
 =========================
 
 You can get the type object from a C++ class that has already been registered using:
 
 .. code-block:: cpp
 
     py::type T_py = py::type::of<T>();
 
 You can directly use ``py::type::of(ob)`` to get the type object from any python
 object, just like ``type(ob)`` in Python.
 
 .. note::
 
     Other types, like ``py::type::of<int>()``, do not work, see :ref:`type-conversions`.
 
 .. versionadded:: 2.6
 
 Custom type setup
 =================
 
 For advanced use cases, such as enabling garbage collection support, you may
 wish to directly manipulate the ``PyHeapTypeObject`` corresponding to a
 ``py::class_`` definition.
 
 You can do that using ``py::custom_type_setup``:
 
 .. code-block:: cpp
 
    struct OwnsPythonObjects {
        py::object value = py::none();
    };
    py::class_<OwnsPythonObjects> cls(
        m, "OwnsPythonObjects", py::custom_type_setup([](PyHeapTypeObject *heap_type) {
            auto *type = &heap_type->ht_type;
            type->tp_flags |= Py_TPFLAGS_HAVE_GC;
            type->tp_traverse = [](PyObject *self_base, visitproc visit, void *arg) {
                auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
                Py_VISIT(self.value.ptr());
                return 0;
            };
            type->tp_clear = [](PyObject *self_base) {
                auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
                self.value = py::none();
                return 0;
            };
        }));
    cls.def(py::init<>());
    cls.def_readwrite("value", &OwnsPythonObjects::value);
 
 .. versionadded:: 2.8

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