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+.. _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:: pycon
+
+ % 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'
+
+.. seealso::
+
+ Static member functions can be bound in the same way using
+ :func:`class_::def_static`.
+
+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 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:: 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
+ 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. 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
+===========
+
+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
+ 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:: 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_));
+
+
+.. [#cpp14] A compiler which supports the ``-std=c++14`` flag
+ or Visual Studio 2015 Update 2 and newer.
+
+.. 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:: pycon
+
+ >>> p = Pet('Lucy', Pet.Cat)
+ >>> p.type
+ Kind.Cat
+ >>> int(p.type)
+ 1L
+
+
+.. 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.
generated by cgit v1.2.3 (git 2.25.1) at 2024-11-20 18:26:26 +0000