# Chapter 2. High Level Components

## boost/python/class.hpp

### 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.

### 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.

Template Parameter | Requirements | Semantics | Default
--- | --- | --- | ---
`T` | A class type. | The class being wrapped |
Bases | A specialization of 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`. | bases<>
HeldType | Must be `T`, a class derived from `T`, or a 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 ptr, `ref`, or Call Policies such as 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.

#### HeldType Semantics

1. 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 init-expression passed to 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.
2. 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>`.
3. 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.
4. Except in cases 1 and 3, users may optionally specify that T itself gets initialized with a similar initial `PyObject*` argument by specializing has_back_reference<T>.

#### 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();
    };
    }}

#### 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);

Requires

  name is an ntbs which conforms to Python's identifier naming rules. If docstring is supplied, it must be an ntbs. If `init_spec` is supplied, it must be either the special enumeration constant `no_init` or an init-expression compatible with `T`.

Effects

  Constructs a `class_` object holding a Boost.Python extension class named name. The named attribute of the 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.

#### Class template `class_` modifier functions

    template <class Init>
    class_& def(Init init_expr);

Requires

  `init_expr` is the result of an init-expression compatible with `T`.

Effects

  For each 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 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);

Requires

  name is an ntbs which conforms to Python's identifier naming rules. * If a1 is the result of an overload-dispatch-expression, only the second form is allowed and fn must be a pointer to function or pointer to member function whose arity is the same as A1's maximum arity.

  Effects: For each prefix `P` of `Fn`'s sequence of argument types, beginning with the one whose length is `A1`'s 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] 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 callable.

      Mnemonic Name | Requirements/Type properties | Effects
      --- | --- | ---
      docstring | Any 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 CallPolicies | A copy will be used as the call policies of the resulting method overload.
      keywords | The result of a keyword-expression specifying no more arguments than the arity of fn. | A copy will be used as the call policies of the resulting method overload.

Returns

  `*this`

    class_& staticmethod(char const* name);

Requires

  name is an ntbs which conforms to Python's 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)))

Note

  Attempting to invoke def(name,...) after invoking staticmethod(name) will raise a RuntimeError.

Returns

  `*this`

    template <unspecified>
    class_& def(detail::operator_<unspecified>);

Effects

  Adds a Python special method as described here.

Returns

  `*this`

    template <class U>
    class_& setattr(char const* name, U const& u);

Requires

  name is an ntbs which conforms to Python's 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);

Requires

  name is an ntbs which conform to Python's identifier naming rules.

Effects

  Creates a new Python 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);

Requires

  name is an ntbs which conforms to Python's 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);

Requires

  name is an ntbs which conforms to Python's identifier naming rules. `doc` is also an 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);

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&);

Requires

  PickleSuite must be publically derived from 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 easy to use high-level interface for establishing complete pickle support for the wrapped class. The user is protected by compile-time consistency checks.

    class_& enable_pickling();

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.

### Class template bases<T1, T2, ...TN>

An MPL sequence which can be used in class_<...> instantiations indicate a list of base classes.

#### Class template bases synopsis

    namespace boost { namespace python
    {
      template <T1 = unspecified,...Tn = unspecified>
      struct bases
      {};
    }}

### 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);