[section boost/python/class.hpp]
[section Introduction]
`<boost/python/class.hpp>` defines the interface through which users expose their C++ classes to Python. It declares the `class_` class template, which is parameterized on the class type being exposed. It also exposes the `init`, `optional` and `bases` utility class templates, which are used in conjunction with `class_`.

`<boost/python/class_fwd.hpp>` contains a forward declaration of the `class_` class template.
[endsect]
[section Class template `class_<T, Bases, HeldType, NonCopyable>`]
Creates a Python class associated with the C++ type passed as its first parameter. Although it has four template parameters, only the first one is required. The three optional arguments can actually be supplied *in any order*\ ; Boost.Python determines the role of the argument from its type.
[table
 [[Template Parameter][Requirements][Semantics][Default]]
 [[`T`][A class type.][The class being wrapped][]]
 [[Bases]
  [A specialization of [link high_level_components.boost_python_class_hpp.class_template_bases_t1_t2_tn bases<...>] which specifies previously-exposed C++ base classes of `T`.]
  [Registers `from_python` conversions from wrapped `T` instances to each of its exposed direct and indirect bases. For each polymorphic base `B`, registers conversions from indirectly-held wrapped `B` instances to `T`.][[link high_level_components.boost_python_class_hpp.class_template_bases_t1_t2_tn bases<>]]]
  [[HeldType][Must be `T`, a class derived from `T`, or a [link concepts.dereferenceable.concept_requirements Dereferenceable] type for which `pointee<HeldType>::type` is `T` or a class derived from `T`.][Specifies the type that is actually embedded in a Python object wrapping a `T` instance when `T`\ 's constructor is called or when a `T` or `T*` is converted to Python without the use of [link function_invocation_and_creation.boost_python_ptr_hpp.functions ptr], `ref`, or [link concepts.callpolicies Call Policies] such as [link function_invocation_and_creation.models_of_callpolicies.boost_python_return_internal_ref.class_template_return_internal_r return_internal_reference]. More details below.][`T`]]
  [[NonCopyable][If supplied, must be `boost::noncopyable`.][Suppresses automatic registration of `to_python` conversions which copy `T` instances. Required when `T` has no publicly-accessible copy constructor.][An unspecified type other than boost::noncopyable.]]
]
[section HeldType Semantics]

# If HeldType is derived from `T`, its exposed constructor(s) must accept an initial `PyObject*` argument which refers back to the Python object that contains the HeldType instance, as shown in this example. This argument is not included in the [link high_level_components.boost_python_init_hpp.introduction.init_expressions init-expression] passed to [link high_level_components.boost_python_class_hpp.class_template_class_t_bases_hel.class_template_class_modifier_fu def(init_expr)], below, nor is it passed explicitly by users when Python instances of `T` are created. This idiom allows C++ virtual functions which will be overridden in Python to access the Python object so the Python method can be invoked. Boost.Python automatically registers additional converters which allow wrapped instances of `T` to be passed to wrapped C++ functions expecting HeldType arguments.
# Because Boost.Python will always allow wrapped instances of `T` to be passed in place of HeldType arguments, specifying a smart pointer for HeldType allows users to pass Python `T` instances where a smart pointer-to-T is expected. Smart pointers such as `std::auto_ptr<>` or `boost::shared_ptr<>` which contain a nested type `element_type` designating the referent type are automatically supported; additional smart pointer types can be supported by specializing `pointee<HeldType>`.
# As in case 1 above, when HeldType is a smart pointer to a class derived from `T`, the initial `PyObject*` argument must be supplied by all of HeldType's exposed constructors.
# Except in cases 1 and 3, users may optionally specify that T itself gets initialized with a similar initial `PyObject*` argument by specializing [link utility_and_infrastructure.boost_python_has_back_reference_.class_template_has_back_referenc has_back_reference<T>].

[endsect]
[section Class template `class_` synopsis]
``
  namespace boost { namespace python
  {
  template <class T
      , class Bases = bases<>
            , class HeldType = T
            , class NonCopyable = unspecified
           >
  class class_ : public object
  {
    // Constructors with default __init__
    class_(char const* name);
    class_(char const* name, char const* docstring);

    // Constructors, specifying non-default __init__
    template <class Init>
    class_(char const* name, Init);
    template <class Init>
    class_(char const* name, char const* docstring, Init);

    // Exposing additional __init__ functions
    template <class Init>
    class_& def(Init);

    // defining methods
    template <class F>
    class_& def(char const* name, F f);
    template <class Fn, class A1>
    class_& def(char const* name, Fn fn, A1 const&);
    template <class Fn, class A1, class A2>
    class_& def(char const* name, Fn fn, A1 const&, A2 const&);
    template <class Fn, class A1, class A2, class A3>
    class_& def(char const* name, Fn fn, A1 const&, A2 const&, A3 const&);

    // declaring method as static
    class_& staticmethod(char const* name);
    
    // exposing operators
    template <unspecified>
    class_& def(detail::operator_<unspecified>);

    // Raw attribute modification
    template <class U>
    class_& setattr(char const* name, U const&);

    // exposing data members
    template <class D>
    class_& def_readonly(char const* name, D T::*pm);

    template <class D>
    class_& def_readwrite(char const* name, D T::*pm);

    // exposing static data members
    template <class D>
    class_& def_readonly(char const* name, D const& d);
    template <class D>
    class_& def_readwrite(char const* name, D& d);

    // property creation
    template <class Get>
    void add_property(char const* name, Get const& fget, char const* doc=0);
    template <class Get, class Set>
    void add_property(
        char const* name, Get const& fget, Set const& fset, char const* doc=0);

    template <class Get>
    void add_static_property(char const* name, Get const& fget);
    template <class Get, class Set>
    void add_static_property(char const* name, Get const& fget, Set const& fset);

    // pickle support
    template <typename PickleSuite>
    self& def_pickle(PickleSuite const&);
    self& enable_pickling();
  };
  }}
``
[endsect]
[section Class template `class_` constructors]
``
  class_(char const* name);
  class_(char const* name, char const* docstring);
  template <class Init>
  class_(char const* name, Init init_spec);
  template <class Init>
  class_(char const* name, char const* docstring, Init init_spec);
``

[variablelist
 [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules]. If docstring is supplied, it must be an [link ntbs]. If `init_spec` is supplied, it must be either the special enumeration constant `no_init` or an [link high_level_components.boost_python_init_hpp.introduction.init_expressions init-expression] compatible with `T`.]]
 [[Effects][Constructs a `class_` object holding a Boost.Python extension class named name. The named attribute of the [link high_level_components.boost_python_scope_hpp.introduction current scope] is bound to the new extension class.

* If supplied, the value of docstring is bound to the `__doc__` attribute of the extension class.
* If `init_spec` is `no_init`, a special `__init__` function is generated which always raises a Python exception. Otherwise, `this->def(init_spec)` is called.
* If `init_spec` is not supplied, `this->def(init<>())` is called.]]
 [[Rationale][Allowing the user to specify constructor arguments in the `class_<>` constructor helps her to avoid the common run-time errors which result from invoking wrapped member functions without having exposed an `__init__` function which creates the requisite `T` instance. Types which are not default-constructible will cause a compile-time error unless `Init` is supplied. The user must always supply name as there is currently no portable method to derive the text of the class name from its type.]]
]
[endsect]
[section Class template `class_` modifier functions]
``
  template <class Init>
  class_& def(Init init_expr);
``

[variablelist
 [[Requires][`init_expr` is the result of an [link high_level_components.boost_python_init_hpp.introduction.init_expressions init-expression] compatible with `T`.]]
 [[Effects][For each [link high_level_components.boost_python_init_hpp.introduction.init_expressions valid prefix] `P` of `Init`, adds an `__init__(...)` function overload to the extension class accepting P as arguments. Each overload generated constructs an object of HeldType according to the semantics described above, using a copy of init_expr's call policies. If the longest [link high_level_components.boost_python_init_hpp.introduction.init_expressions valid prefix] of Init contains N types and init_expr holds M keywords, an initial sequence of the keywords are used for all but the first N - M arguments of each overload.]]
 [[Returns][`*this`]]
 [[Rationale][Allows users to easily expose a class' constructor to Python.]] 
]
``
  template <class F>
  class_& def(char const* name, Fn fn);
  template <class Fn, class A1>
  class_& def(char const* name, Fn fn, A1 const& a1);
  template <class Fn, class A1, class A2>
  class_& def(char const* name, Fn fn, A1 const& a1, A2 const& a2);
  template <class Fn, class A1, class A2, class A3>
  class_& def(char const* name, Fn fn, A1 const& a1, A2 const& a2, A3 const& a3);
``
[variablelist
 [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules].
 * If a1 is the result of an [link function_invocation_and_creation.boost_python_overloads_hpp.introduction.overload_dispatch_expressions overload-dispatch-expression], only the second form is allowed and fn must be a pointer to function or pointer to member function whose [link arity] is the same as A1's [link function_invocation_and_creation.boost_python_overloads_hpp.introduction.overload_dispatch_expressions maximum arity].

   [*Effects:] For each prefix `P` of `Fn`\ 's sequence of argument types, beginning with the one whose length is `A1`\ 's [link function_invocation_and_creation.boost_python_overloads_hpp.introduction.overload_dispatch_expressions minimum arity], adds a `name(...)` method overload to the extension class. Each overload generated invokes a1's call-expression with `P`, using a copy of a1's call policies. If the longest valid prefix of `A1` contains `N` types and a1 holds `M` keywords, an initial sequence of the keywords are used for all but the first `N - M` arguments of each overload.

* Otherwise, a single method overload is built around fn, which must not be null:

    * If fn is a function pointer, its first argument must be of the form U, U cv&, U cv*, or U cv* const&, where T* is convertible to U*, and a1-a3, if supplied, may be selected in any order from the table below.
    * Otherwise, if fn is a member function pointer, its target must be T or one of its public base classes, and a1-a3, if supplied, may be selected in any order from the table below.
    * Otherwise, Fn must be [derived from] [link object_wrappers.boost_python_object_hpp.class_object object], and a1-a2, if supplied, may be selcted in any order from the first two rows of the table below. To be useful, fn should be [@http://www.python.org/doc/current/lib/built-in-funcs.html#l2h-6 callable].
  [table
    [[Mnemonic Name][Requirements/Type properties][Effects]]
    [[docstring][Any [link ntbs]][Value will be bound to the __doc__ attribute of the resulting method overload. If an earlier overload supplied a docstring, two newline characters and the new docstring are appended to it.]]
    [[policies][A model of [link concepts.callpolicies CallPolicies]][A copy will be used as the call policies of the resulting method overload.]]
    [[keywords][The result of a [link function_invocation_and_creation.boost_python_args_hpp.introduction.keyword_expressions keyword-expression] specifying no more arguments than the [link arity] of fn.][A copy will be used as the call policies of the resulting method overload.]]
  ]
]]
 [[Returns][`*this`]]
]
``class_& staticmethod(char const* name);``
[variablelist
  [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules], and corresponds to a method whose overloads have all been defined.]]
  [[Effects][Replaces the existing named attribute `x` with the result of invoking `staticmethod(x)` in Python. Specifies that the corresponding method is static and therefore no object instance will be passed to it. This is equivalent to the Python statement:]]
]
``setattr(self, name, staticmethod(getattr(self, name)))``
[variablelist
 [[Note][Attempting to invoke def(name,...) after invoking staticmethod(name) will [link raise] a RuntimeError.]]
 [[Returns][`*this`]]
]
``
template <unspecified>
class_& def(detail::operator_<unspecified>);
``
[variablelist
  [[Effects][Adds a Python [@http://www.python.org/doc/ref/specialnames.html special method] as described [link high_level_components.boost_python_operators_hpp here].]]
  [[Returns][`*this`]]
]

``
  template <class U>
  class_& setattr(char const* name, U const& u);
``
[variablelist
  [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules].]]
  [[Effects][Converts `u` to Python and adds it to the attribute dictionary of the extension class:
  ``PyObject_SetAttrString(this->ptr(), name, object(u).ptr());``]]
  [[Returns][`*this`]]
]
``
  template <class Get>
  void add_property(char const* name, Get const& fget, char const* doc=0);
  template <class Get, class Set>
  void add_property(
        char const* name, Get const& fget, Set const& fset, char const* doc=0);
``
[variablelist
  [[Requires][name is an [link ntbs] which conform to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules].]]
  [[Effects][Creates a new Python [@http://www.python.org/2.2.2/descrintro.html#property property] class instance, passing `object(fget)` (and `object(fset)` in the second form) with an (optional) docstring `doc` to its constructor, then adds that property to the Python class object under construction with the given attribute name.]]
  [[Returns][`*this`]]
  [[Rationale][Allows users to easily expose functions that can be invoked from Python with attribute access syntax.]]
]
``
  template <class Get>
  void add_static_property(char const* name, Get const& fget);
  template <class Get, class Set>
  void add_static_property(char const* name, Get const& fget, Set const& fset);
``
[variablelist
  [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules].]]
  [[Effects][Creates a Boost.Python.StaticProperty object, passing `object(fget)` (and `object(fset)` in the second form) to its constructor, then adds that property to the Python class under construction with the given attribute name. StaticProperty is a special subclass of Python's property class which can be called without an initial self argument.]]
  [[Returns][`*this`]]
  [[Rationale][Allows users to easily expose functions that can be invoked from Python with static attribute access syntax.]]
]
``
  template <class D>
  class_& def_readonly(char const* name, D T::*pm, char const* doc=0);
  template <class D>
  class_& def_readonly(char const* name, D const& d);
``
[variablelist
  [[Requires][name is an [link ntbs] which conforms to Python's [@http://www.python.org/doc/current/ref/identifiers.html identifier naming rules]. `doc` is also an [link ntbs].]]
  [[Effects][``this->add_property(name, make_getter(pm), doc);`` and ``this->add_static_property(name, make_getter(d));`` respectively.]]
  [[Returns][`*this`]]
  [[Rationale][Allows users to easily expose a class' data member or free variable such that it can be inspected from Python with a natural syntax.]]
]
``
  template <class D>
  class_& def_readwrite(char const* name, D T::*pm, char const* doc=0);
  template <class D>
  class_& def_readwrite(char const* name, D& d);
``
[variablelist
[[Effects][``this->add_property(name, make_getter(pm), make_setter(pm), doc);`` and ``this->add_static_property(name, make_getter(d), make_setter(d));`` respectively.]]
[[Returns][`*this`]]
[[Rationale][Allows users to easily expose a class' data or free variable member such that it can be inspected and set from Python with a natural syntax.]]
]
``
  template <typename PickleSuite>
  class_& def_pickle(PickleSuite const&);
``
[variablelist
[[Requires][PickleSuite must be publically derived from [link topics.pickle_support.the_pickle_interface pickle_suite].]]
[[Effects][Defines a legal combination of the special attributes and methods: __getinitargs__, __getstate__, __setstate__, __getstate_manages_dict__, __safe_for_unpickling__, __reduce__]]
[[Returns][`*this`]]
[[Rationale][Provides an [link topics.pickle_support.the_pickle_interface easy to use high-level interface] for establishing complete [link topics.pickle_support.the_pickle_interface pickle support] for the wrapped class. The user is protected by compile-time consistency checks.]]
]
``class_& enable_pickling();``
[variablelist
[[Effects][Defines the __reduce__ method and the __safe_for_unpickling__ attribute.]]
[[Returns][`*this`]]
[[Rationale][Light-weight alternative to def_pickle(). Enables implementation of pickle support from Python.]]
]
[endsect]
[endsect]
[section Class template bases<T1, T2, ...TN>]
An MPL sequence which can be used in class_<...> instantiations indicate a list of base classes.
[section Class template bases synopsis]
``
namespace boost { namespace python
{
  template <T1 = unspecified,...Tn = unspecified>
  struct bases
  {};
}}
``
[endsect]
[endsect]
[section Examples]
Given a C++ class declaration:
``
class Foo : public Bar, public Baz
{
 public:
   Foo(int x, char const* y);
   Foo(double);

   std::string const& name() { return m_name; }
   void name(char const*);

   double value; // public data
 private:
   ...
};
``
A corresponding Boost.Python extension class can be created with:
``
using namespace boost::python;

class_<Foo,bases<Bar,Baz> >("Foo",
          "This is Foo's docstring."
          "It describes our Foo extension class",

          init<int,char const*>(args("x","y"), "__init__ docstring")
          )
   .def(init<double>())
   .def("get_name", &Foo::get_name, return_internal_reference<>())
   .def("set_name", &Foo::set_name)
   .def_readwrite("value", &Foo::value);
``
[endsect]
[endsect]