116 lines
3.2 KiB
Plaintext
116 lines
3.2 KiB
Plaintext
[section boost/python/has_back_reference.hpp]
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[section Introduction]
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<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.
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[endsect]
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[section Class template `has_back_reference`]
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A unary metafunction whose value is true iff its argument is a `pointer_wrapper<>`.
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``
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namespace boost { namespace python
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{
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template<class WrappedClass> class has_back_reference
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{
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typedef mpl::false_ type;
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};
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}}
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``
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A metafunction that is inspected by Boost.Python to determine how wrapped classes can be constructed.
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`type::value` is an integral constant convertible to bool of unspecified type.
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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_`.
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[endsect]
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[section Examples]
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In C++:
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``
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#include <boost/python/class.hpp>
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#include <boost/python/module.hpp>
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#include <boost/python/has_back_reference.hpp>
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#include <boost/python/handle.hpp>
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#include <boost/shared_ptr.hpp>
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using namespace boost::python;
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using boost::shared_ptr;
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struct X
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{
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X(PyObject* self) : m_self(self), m_x(0) {}
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X(PyObject* self, int x) : m_self(self), m_x(x) {}
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X(PyObject* self, X const& other) : m_self(self), m_x(other.m_x) {}
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handle<> self() { return handle<>(borrowed(m_self)); }
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int get() { return m_x; }
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void set(int x) { m_x = x; }
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PyObject* m_self;
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int m_x;
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};
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// specialize has_back_reference for X
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namespace boost { namespace python
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{
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template <>
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struct has_back_reference<X>
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: mpl::true_
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{};
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}}
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struct Y
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{
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Y() : m_x(0) {}
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Y(int x) : m_x(x) {}
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int get() { return m_x; }
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void set(int x) { m_x = x; }
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int m_x;
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};
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shared_ptr<Y>
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Y_self(shared_ptr<Y> self) { return self; }
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BOOST_PYTHON_MODULE(back_references)
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{
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class_<X>("X")
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.def(init<int>())
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.def("self", &X::self)
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.def("get", &X::get)
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.def("set", &X::set)
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;
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class_<Y, shared_ptr<Y> >("Y")
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.def(init<int>())
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.def("get", &Y::get)
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.def("set", &Y::set)
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.def("self", Y_self)
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;
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}
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``
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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.
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In Python:
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``
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>>> from back_references import *
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>>> x = X(1)
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>>> x2 = x.self()
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>>> x2 is x
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1
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>>> (x.get(), x2.get())
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(1, 1)
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>>> x.set(10)
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>>> (x.get(), x2.get())
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(10, 10)
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>>>
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>>>
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>>> y = Y(2)
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>>> y2 = y.self()
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>>> y2 is y
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0
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>>> (y.get(), y2.get())
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(2, 2)
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>>> y.set(20)
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>>> (y.get(), y2.get())
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(20, 20)
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``
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[endsect]
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[endsect]
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