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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
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_OVERLOAD_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_OVERLOAD_PURE` should be used for pure virtual
functions, and :c:macro:`PYBIND11_OVERLOAD` should be used for functions which have
a default implementation.  There are also two alternate macros 
:c:macro:`PYBIND11_OVERLOAD_PURE_NAME` and :c:macro:`PYBIND11_OVERLOAD_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 overload 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)
    u'woof! woof! woof! '
    >>> class Cat(Animal):
...     def go(self, n_times):
...             return "meow! " * n_times
...
    >>> c = Cat()
>>> call_go(c)
            u'meow!        u'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.

Here is an example:

.. code-block:: python

    class Dachshund(Dog):
        def __init__(self, name):
            Dog.__init__(self) # Without this, undefined behavior may occur if the C++ portions are referenced.
            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_OVERLOAD macros (when needed) with static storage duration.
      Note that this means that invoking the overloaded 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 overload is invoked.

.. 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_OVERLOAD_PURE(std::string, Animal, go, n_times); }
        std::string name() override { PYBIND11_OVERLOAD(std::string, Animal, name, ); }
    };
    class PyDog : public Dog {
    public:
        using Dog::Dog; // Inherit constructors
        std::string go(int n_times) override { PYBIND11_OVERLOAD(std::string, Dog, go, n_times); }
        std::string name() override { PYBIND11_OVERLOAD(std::string, Dog, name, ); }
        std::string bark() override { PYBIND11_OVERLOAD(std::string, Dog, bark, ); }
    };

.. note::

    Note the trailing commas in the ``PYBIND11_OVERLOAD`` 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_OVERLOAD_PURE(std::string, Husky, go, n_times); }
        std::string name() override { PYBIND11_OVERLOAD(std::string, Husky, name, ); }
        std::string bark() override { PYBIND11_OVERLOAD(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_OVERLOAD_PURE(std::string, AnimalBase, go, n_times); }
        std::string name() override { PYBIND11_OVERLOAD(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_OVERLOAD(std::string, DogBase, go, n_times); }
        std::string bark() override { PYBIND11_OVERLOAD(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, PyDog<>> dog(m, "Dog");
    py::class_<Husky, 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 overloaded 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_overload`, 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 overloaded method on the Python side.
        pybind11::function overload = pybind11::get_overload(this, "myMethod");
        if (overload) {  // method is found
            auto obj = overload(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")


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