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Boost.PythonHeader <boost/python/has_back_reference.hpp> |
has_back_referencehas_back_reference synopsis<boost/python/has_back_reference.hpp> defines the
predicate metafunction has_back_reference<>, which can
be specialized by the user to indicate that a wrapped class instance
holds a PyObject* corresponding to a Python object.
has_back_referenceA unary metafunction whose value is true iff its argument
is a pointer_wrapper<>.
has_back_reference synopsis
namespace boost { namespace python
{
template<class WrappedClass> class has_back_reference
{
typedef mpl::false_ type;
};
}}
A " metafunction" that is inspected by Boost.Python to determine how wrapped classes can be constructed.
type::value is an integral constant convertible to bool
of unspecified type.true-valued integral constant wrapper for
type iff for each invocation of
class_<WrappedClass>::def(init<
type-sequence...>()) and the implicitly wrapped
copy constructor (unless it is
noncopyable), there exists a corresponding constructor
WrappedClass::WrappedClass(PyObject*,
type-sequence...). If such a specialization exists,
the WrappedClass constructors will be called with a "back
reference" pointer to the corresponding Python object whenever they are
invoked from Python. The easiest way to provide this nested
type
is to
derive the specialization from mpl::true_.
#include <boost/python/class.hpp>
#include <boost/python/module.hpp>
#include <boost/python/has_back_reference.hpp>
#include <boost/python/handle.hpp>
#include <boost/shared_ptr.hpp>
using namespace boost::python;
using boost::shared_ptr;
struct X
{
X(PyObject* self) : m_self(self), m_x(0) {}
X(PyObject* self, int x) : m_self(self), m_x(x) {}
X(PyObject* self, X const& other) : m_self(self), m_x(other.m_x) {}
handle<> self() { return handle<>(borrowed(m_self)); }
int get() { return m_x; }
void set(int x) { m_x = x; }
PyObject* m_self;
int m_x;
};
// specialize has_back_reference for X
namespace boost { namespace python
{
template <>
struct has_back_reference<X>
: mpl::true_
{};
}}
struct Y
{
Y() : m_x(0) {}
Y(int x) : m_x(x) {}
int get() { return m_x; }
void set(int x) { m_x = x; }
int m_x;
};
shared_ptr<Y>
Y_self(shared_ptr<Y> self) { return self; }
BOOST_PYTHON_MODULE(back_references)
{
class_<X>("X")
.def(init<int>())
.def("self", &X::self)
.def("get", &X::get)
.def("set", &X::set)
;
class_<Y, shared_ptr<Y> >("Y")
.def(init<int>())
.def("get", &Y::get)
.def("set", &Y::set)
.def("self", Y_self)
;
}
The following Python session illustrates that x.self()
returns the same Python object on which it is invoked, while
y.self() must create a new Python object which refers to the
same Y instance.
>>> from back_references import * >>> x = X(1) >>> x2 = x.self() >>> x2 is x 1 >>> (x.get(), x2.get()) (1, 1) >>> x.set(10) >>> (x.get(), x2.get()) (10, 10) >>> >>> >>> y = Y(2) >>> y2 = y.self() >>> y2 is y 0 >>> (y.get(), y2.get()) (2, 2) >>> y.set(20) >>> (y.get(), y2.get()) (20, 20)
Revised 18 July, 2004
© Copyright Dave Abrahams 2002 .