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

: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:: pycon

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

.. seealso::

Static member functions can be bound in the same way using
:func:`class_::def_static`.

.. note::

Binding C++ types in unnamed namespaces (also known as anonymous namespaces)
works reliably on many platforms, but not all. The `XFAIL_CONDITION` in
tests/test_unnamed_namespace_a.py encodes the currently known conditions.
For background see `#4319 <https://github.com/pybind/pybind11/pull/4319>`_.
If portability is a concern, it is therefore not recommended to bind C++
types in unnamed namespaces. It will be safest to manually pick unique
namespace names.

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:: pycon

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

To address this, we could bind a 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:: pycon

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

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

.. _properties:

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:: pycon

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

Write only properties can be defined by passing nullptr as the input for the read function.

.. 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. Please also see the section on
:ref:`static_properties` in the advanced part of the documentation.

Dynamic attributes

Native Python classes can pick up new attributes dynamically:

.. code-block:: pycon

>>> class Pet:
...     name = "Molly"
...
>>> p = Pet()
>>> p.name = "Charly"  # overwrite existing
>>> p.age = 2  # dynamically add a new attribute

By default, classes exported from C++ do not support this and the only writable attributes are the ones explicitly defined using :func:class_::def_readwrite or :func:class_::def_property.

.. code-block:: cpp

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

Trying to set any other attribute results in an error:

.. code-block:: pycon

>>> p = example.Pet()
>>> p.name = "Charly"  # OK, attribute defined in C++
>>> p.age = 2  # fail
AttributeError: 'Pet' object has no attribute 'age'

To enable dynamic attributes for C++ classes, the :class:py::dynamic_attr tag must be added to the :class:py::class_ constructor:

.. code-block:: cpp

py::class_<Pet>(m, "Pet", py::dynamic_attr())
    .def(py::init<>())
    .def_readwrite("name", &Pet::name);

Now everything works as expected:

.. code-block:: pycon

>>> p = example.Pet()
>>> p.name = "Charly"  # OK, overwrite value in C++
>>> p.age = 2  # OK, dynamically add a new attribute
>>> p.__dict__  # just like a native Python class
{'age': 2}

Note that there is a small runtime cost for a class with dynamic attributes. Not only because of the addition of a __dict__, but also because of more expensive garbage collection tracking which must be activated to resolve possible circular references. Native Python classes incur this same cost by default, so this is not anything to worry about. By default, pybind11 classes are more efficient than native Python classes. Enabling dynamic attributes just brings them on par.

.. _inheritance:

Inheritance and automatic downcasting

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!"; }
};

There are two different ways of indicating a hierarchical relationship to pybind11: the first specifies the C++ base class as an extra template parameter of the :class:class_:

.. code-block:: cpp

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

// Method 1: template parameter:
py::class_<Dog, Pet /* <- specify C++ parent type */>(m, "Dog")
    .def(py::init<const std::string &>())
    .def("bark", &Dog::bark);

Alternatively, we can also assign a name to the previously bound Pet :class:class_ object 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);

// Method 2: pass parent class_ object:
py::class_<Dog>(m, "Dog", pet /* <- specify Python parent type */)
    .def(py::init<const std::string &>())
    .def("bark", &Dog::bark);

Functionality-wise, both approaches are equivalent. Afterwards, instances will expose fields and methods of both types:

.. code-block:: pycon

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

The C++ classes defined above are regular non-polymorphic types with an inheritance relationship. This is reflected in Python:

.. code-block:: cpp

// Return a base pointer to a derived instance
m.def("pet_store", []() { return std::unique_ptr<Pet>(new Dog("Molly")); });

.. code-block:: pycon

>>> p = example.pet_store()
>>> type(p)  # `Dog` instance behind `Pet` pointer
Pet          # no pointer downcasting for regular non-polymorphic types
>>> p.bark()
AttributeError: 'Pet' object has no attribute 'bark'

The function returned a Dog instance, but because it's a non-polymorphic type behind a base pointer, Python only sees a Pet. In C++, a type is only considered polymorphic if it has at least one virtual function and pybind11 will automatically recognize this:

.. code-block:: cpp

struct PolymorphicPet {
    virtual ~PolymorphicPet() = default;
};

struct PolymorphicDog : PolymorphicPet {
    std::string bark() const { return "woof!"; }
};

// Same binding code
py::class_<PolymorphicPet>(m, "PolymorphicPet");
py::class_<PolymorphicDog, PolymorphicPet>(m, "PolymorphicDog")
    .def(py::init<>())
    .def("bark", &PolymorphicDog::bark);

// Again, return a base pointer to a derived instance
m.def("pet_store2", []() { return std::unique_ptr<PolymorphicPet>(new PolymorphicDog); });

.. code-block:: pycon

>>> p = example.pet_store2()
>>> type(p)
PolymorphicDog  # automatically downcast
>>> p.bark()
'woof!'

Given a pointer to a polymorphic base, pybind11 performs automatic downcasting to the actual derived type. Note that this goes beyond the usual situation in C++: we don't just get access to the virtual functions of the base, we get the concrete derived type including functions and attributes that the base type may not even be aware of.

.. seealso::

For more information about polymorphic behavior see :ref:`overriding_virtuals`.

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", static_cast<void (Pet::*)(int)>(&Pet::set), "Set the pet's age")
   .def("set", static_cast<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:: pycon

>>> help(example.Pet)

class Pet(__builtin__.object)
 |  Methods defined here:
 |
 |  __init__(...)
 |      Signature : (Pet, str, int) -> NoneType
 |
 |  set(...)
 |      1. Signature : (Pet, int) -> NoneType
 |
 |      Set the pet's age
 |
 |      2. Signature : (Pet, str) -> NoneType
 |
 |      Set the pet's name

If you have a C++14 compatible compiler [#cpp14]_, you can use an alternative syntax to cast the overloaded function:

.. code-block:: cpp

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

Here, py::overload_cast only requires the parameter types to be specified. The return type and class are deduced. This avoids the additional noise of void (Pet::*)() as seen in the raw cast. If a function is overloaded based on constness, the py::const_ tag should be used:

.. code-block:: cpp

struct Widget {
    int foo(int x, float y);
    int foo(int x, float y) const;
};

py::class_<Widget>(m, "Widget")
   .def("foo_mutable", py::overload_cast<int, float>(&Widget::foo))
   .def("foo_const",   py::overload_cast<int, float>(&Widget::foo, py::const_));

If you prefer the py::overload_cast syntax but have a C++11 compatible compiler only, you can use py::detail::overload_cast_impl with an additional set of parentheses:

.. code-block:: cpp

template <typename... Args>
using overload_cast_ = pybind11::detail::overload_cast_impl<Args...>;

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

.. [#cpp14] A compiler which supports the -std=c++14 flag.

.. 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 internal types like enumerations, e.g.:

.. code-block:: cpp

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

    struct Attributes {
        float age = 0;
    };

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

    std::string name;
    Kind type;
    Attributes attr;
};

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)
    .def_readwrite("attr", &Pet::attr);

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

py::class_<Pet::Attributes>(pet, "Attributes")
    .def(py::init<>())
    .def_readwrite("age", &Pet::Attributes::age);

To ensure that the nested types Kind and Attributes are created within the scope of Pet, the pet :class:class_ instance must be supplied to the :class:enum_ and :class:class_ 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:: pycon

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

The entries defined by the enumeration type are exposed in the __members__ property:

.. code-block:: pycon

>>> Pet.Kind.__members__
{'Dog': Kind.Dog, 'Cat': Kind.Cat}

The name property returns the name of the enum value as a unicode string.

.. note::

It is also possible to use ``str(enum)``, however these accomplish different
goals. The following shows how these two approaches differ.

.. code-block:: pycon

    >>> p = Pet("Lucy", Pet.Cat)
    >>> pet_type = p.type
    >>> pet_type
    Pet.Cat
    >>> str(pet_type)
    'Pet.Cat'
    >>> pet_type.name
    'Cat'

.. note::

When the special tag ``py::arithmetic()`` is specified to the ``enum_``
constructor, pybind11 creates an enumeration that also supports rudimentary
arithmetic and bit-level operations like comparisons, and, or, xor, negation,
etc.

.. code-block:: cpp

    py::enum_<Pet::Kind>(pet, "Kind", py::arithmetic())
       ...

By default, these are omitted to conserve space.

.. warning::

Contrary to Python customs, enum values from the wrappers should not be compared using ``is``, but with ``==`` (see `#1177 <https://github.com/pybind/pybind11/issues/1177>`_ for background).