...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Table of Contents
<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.
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 |
---|---|---|---|
|
A class type. |
The class being wrapped |
|
Bases |
A specialization of bases<...>
which specifies previously-exposed C++ base classes of |
Registers |
|
HeldType |
Must be |
Specifies the type that is actually embedded in a Python object
wrapping a |
|
NonCopyable |
If supplied, must be |
Suppresses automatic registration of |
An unspecified type other than boost::noncopyable. |
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.
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>
.
T
, the initial
PyObject*
argument must be supplied by all of HeldType's exposed constructors.
PyObject*
argument by specializing has_back_reference<T>.
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_(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);
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
.
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.
__doc__
attribute of the
extension class.
init_spec
is no_init
,
a special __init__
function is generated which always raises a Python exception.
Otherwise, this->def(init_spec)
is called.
init_spec
is not supplied, this->def(init<>())
is called.
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.
template <class Init> class_& def(Init init_expr);
init_expr
is the
result of an init-expression
compatible with T
.
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.
*this
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);
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, 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. |
*this
class_& staticmethod(char const* name);
name is an ntbs which conforms to Python's identifier naming rules, and corresponds to a method whose overloads have all been defined.
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)))
Attempting to invoke def(name,...) after invoking staticmethod(name) will raise a RuntimeError.
*this
template <unspecified> class_& def(detail::operator_<unspecified>);
Adds a Python special method as described here.
*this
template <class U> class_& setattr(char const* name, U const& u);
name is an ntbs which conforms to Python's identifier naming rules.
Converts u
to Python
and adds it to the attribute dictionary of the extension class:
PyObject_SetAttrString(this->ptr(), name, object(u).ptr());
*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);
name is an ntbs which conform to Python's identifier naming rules.
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.
*this
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);
name is an ntbs which conforms to Python's identifier naming rules.
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.
*this
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);
name is an ntbs which conforms to Python's
identifier
naming rules. doc
is also an ntbs.
this->add_property(name, make_getter(pm), doc);
and
this->add_static_property(name, make_getter(d));
respectively.
*this
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);
this->add_property(name, make_getter(pm), make_setter(pm), doc);
and
this->add_static_property(name, make_getter(d), make_setter(d));
respectively.
*this
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&);
PickleSuite must be publically derived from pickle_suite.
Defines a legal combination of the special attributes and methods: __getinitargs__, __getstate__, __setstate__, __getstate_manages_dict__, __safe_for_unpickling__, __reduce__
*this
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();
Defines the __reduce__ method and the __safe_for_unpickling__ attribute.
*this
Light-weight alternative to def_pickle(). Enables implementation of pickle support from Python.
An MPL sequence which can be used in class_<...> instantiations indicate a list of base classes.
namespace boost { namespace python { template <T1 = unspecified,...Tn = unspecified> struct bases {}; }}
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);