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- [section boost/python/has_back_reference.hpp]
- [section Introduction]
- <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.
- [endsect]
- [section Class template `has_back_reference`]
- A unary metafunction whose value is true iff its argument is a `pointer_wrapper<>`.
- ``
- 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.
- Specializations may substitute a 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_`.
- [endsect]
- [section Examples]
- In C++:
- ``
- #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.
- In Python:
- ``
- >>> 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)
- ``
- [endsect]
- [endsect]
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