classes.rst

.. _classes:

Object-oriented code
####################

Creating bindings for a custom type
===================================

Let's now look at a more complex example where we'll create bindings for a
custom C++ data structure named ``Pet``. Its definition is given below:

.. code-block:: cpp

    struct Pet {
        Pet(const std::string &name) : name(name) { }
        void setName(const std::string &name_) { name = name_; }
        const std::string &getName() const { return name; }

        std::string name;
    };

The binding code for ``Pet`` looks as follows:

.. code-block:: cpp

    #include <pybind11/pybind11.h>

    namespace py = pybind11;

    PYBIND11_PLUGIN(example) {
        py::module m("example", "pybind11 example plugin");

        py::class_<Pet>(m, "Pet")
            .def(py::init<const std::string &>())
            .def("setName", &Pet::setName)
            .def("getName", &Pet::getName);

        return m.ptr();
    }

:class:`class_` creates bindings for a C++ `class` or `struct`-style data
structure. :func:`init` is a convenience function that takes the types of a
constructor's parameters as template arguments and wraps the corresponding
constructor (see the :ref:`custom_constructors` section for details). An
interactive Python session demonstrating this example is shown below:

.. code-block:: python

    % python
    >>> import example
    >>> p = example.Pet('Molly')
    >>> print(p)
    <example.Pet object at 0x10cd98060>
    >>> p.getName()
    u'Molly'
    >>> p.setName('Charly')
    >>> p.getName()
    u'Charly'

Keyword and default arguments
=============================
It is possible to specify keyword and default arguments using the syntax
discussed in the previous chapter. Refer to the sections :ref:`keyword_args`
and :ref:`default_args` for details.

Binding lambda functions
========================

Note how ``print(p)`` produced a rather useless summary of our data structure in the example above:

.. code-block:: python

    >>> print(p)
    <example.Pet object at 0x10cd98060>

To address this, we could bind an utility function that returns a human-readable
summary to the special method slot named ``__repr__``. Unfortunately, there is no
suitable functionality in the ``Pet`` data structure, and it would be nice if
we did not have to change it. This can easily be accomplished by binding a
Lambda function instead:

.. code-block:: cpp

        py::class_<Pet>(m, "Pet")
            .def(py::init<const std::string &>())
            .def("setName", &Pet::setName)
            .def("getName", &Pet::getName)
            .def("__repr__",
                [](const Pet &a) {
                    return "<example.Pet named '" + a.name + "'>";
                }
            );

Both stateless [#f1]_ and stateful lambda closures are supported by pybind11.
With the above change, the same Python code now produces the following output:

.. code-block:: python

    >>> print(p)
    <example.Pet named 'Molly'>

Instance and static fields
==========================

We can also directly expose the ``name`` field using the
:func:`class_::def_readwrite` method. A similar :func:`class_::def_readonly`
method also exists for ``const`` fields.

.. code-block:: cpp

        py::class_<Pet>(m, "Pet")
            .def(py::init<const std::string &>())
            .def_readwrite("name", &Pet::name)
            // ... remainder ...

This makes it possible to write

.. code-block:: python

    >>> p = example.Pet('Molly')
    >>> p.name
    u'Molly'
    >>> p.name = 'Charly'
    >>> p.name
    u'Charly'

Now suppose that ``Pet::name`` was a private internal variable
that can only be accessed via setters and getters.

.. code-block:: cpp

    class Pet {
    public:
        Pet(const std::string &name) : name(name) { }
        void setName(const std::string &name_) { name = name_; }
        const std::string &getName() const { return name; }
    private:
        std::string name;
    };

In this case, the method :func:`class_::def_property`
(:func:`class_::def_property_readonly` for read-only data) can be used to
provide a field-like interface within Python that will transparently call
the setter and getter functions:

.. code-block:: cpp

        py::class_<Pet>(m, "Pet")
            .def(py::init<const std::string &>())
            .def_property("name", &Pet::getName, &Pet::setName)
            // ... remainder ...

.. seealso::

    Similar functions :func:`class_::def_readwrite_static`,
    :func:`class_::def_readonly_static` :func:`class_::def_property_static`,
    and :func:`class_::def_property_readonly_static` are provided for binding
    static variables and properties.

Inheritance
===========

Suppose now that the example consists of two data structures with an
inheritance relationship:

.. code-block:: cpp

    struct Pet {
        Pet(const std::string &name) : name(name) { }
        std::string name;
    };

    struct Dog : Pet {
        Dog(const std::string &name) : Pet(name) { }
        std::string bark() const { return "woof!"; }
    };

To capture the hierarchical relationship in pybind11, we must assign a name to
the ``Pet`` :class:`class_` instance and reference it when binding the ``Dog``
class.

.. code-block:: cpp

    py::class_<Pet> pet(m, "Pet");
    pet.def(py::init<const std::string &>())
       .def_readwrite("name", &Pet::name);

    py::class_<Dog>(m, "Dog", pet /* <- specify parent */)
        .def(py::init<const std::string &>())
        .def("bark", &Dog::bark);

Instances then expose fields and methods of both types:

.. code-block:: python

    >>> p = example.Dog('Molly')
    >>> p.name
    u'Molly'
    >>> p.bark()
    u'woof!'

Overloaded methods
==================

Sometimes there are several overloaded C++ methods with the same name taking
different kinds of input arguments:

.. code-block:: cpp

    struct Pet {
        Pet(const std::string &name, int age) : name(name), age(age) { }

        void set(int age) { age = age; }
        void set(const std::string &name) { name = name; }

        std::string name;
        int age;
    };

Attempting to bind ``Pet::set`` will cause an error since the compiler does not
know which method the user intended to select. We can disambiguate by casting
them to function pointers. Binding multiple functions to the same Python name
automatically creates a chain of function overloads that will be tried in
sequence.

.. code-block:: cpp

    py::class_<Pet>(m, "Pet")
       .def(py::init<const std::string &, int>())
       .def("set", (void (Pet::*)(int)) &Pet::set, "Set the pet's age")
       .def("set", (void (Pet::*)(const std::string &)) &Pet::set, "Set the pet's name");

The overload signatures are also visible in the method's docstring:

.. code-block:: python

    >>> help(example.Pet)

    class Pet(__builtin__.object)
     |  Methods defined here:
     |
     |  __init__(...)
     |      Signature : (Pet, str, int32_t) -> None
     |
     |  set(...)
     |      1. Signature : (Pet, int32_t) -> None
     |
     |      Set the pet's age
     |
     |      2. Signature : (Pet, str) -> None
     |
     |      Set the pet's name

.. note::

    To define multiple overloaded constructors, simply declare one after the
    other using the ``.def(py::init<...>())`` syntax. The existing machinery
    for specifying keyword and default arguments also works.

Enumerations and internal types
===============================

Let's now suppose that the example class contains an internal enumeration type,
e.g.:

.. code-block:: cpp

    struct Pet {
        enum Kind {
            Dog = 0,
            Cat
        };

        Pet(const std::string &name, Kind type) : name(name), type(type) { }

        std::string name;
        Kind type;
    };

The binding code for this example looks as follows:

.. code-block:: cpp

    py::class_<Pet> pet(m, "Pet");

    pet.def(py::init<const std::string &, Pet::Kind>())
        .def_readwrite("name", &Pet::name)
        .def_readwrite("type", &Pet::type);

    py::enum_<Pet::Kind>(pet, "Kind")
        .value("Dog", Pet::Kind::Dog)
        .value("Cat", Pet::Kind::Cat)
        .export_values();

To ensure that the ``Kind`` type is created within the scope of ``Pet``, the
``pet`` :class:`class_` instance must be supplied to the :class:`enum_`.
constructor. The :func:`enum_::export_values` function exports the enum entries
into the parent scope, which should be skipped for newer C++11-style strongly
typed enums.

.. code-block:: python

    >>> p = Pet('Lucy', Pet.Cat)
    >>> p.type
    Kind.Cat
    >>> int(p.type)
    1L


.. [#f1] Stateless closures are those with an empty pair of brackets ``[]`` as the capture object.