boost-react-native-bundle

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">    <html> <head> <meta name="generator" content= "HTML Tidy for Windows (vers 1st August 2002), see www.w3.org"> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" type="text/css" href="../boost.css"> <title>Boost.Python - <boost/python/has_back_reference.hpp></title> <style type="text/css"> p.c3 {font-style: italic} h2.c2 {text-align: center} h1.c1 {text-align: center} </style> </head> <body> <table border="0" cellpadding="7" cellspacing="0" width="100%" summary= "header"> <tr> <td valign="top" width="300"> <h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3> </td> <td valign="top"> <h1 class="c1"><a href="../index.html">Boost.Python</a></h1> <h2 class="c2">Header <boost/python/has_back_reference.hpp></h2> </td> </tr> </table> <hr> <h2>Contents</h2> <dl class="page-index"> <dt><a href="#introduction">Introduction</a></dt> <dt><a href="#classes">Classes</a></dt> <dd> <dl class="page-index"> <dt><a href="#has_back_reference-spec">Class template <code>has_back_reference</code></a></dt> <dd> <dl class="page-index"> <dt><a href="#has_back_reference-spec-synopsis">Class template <code>has_back_reference</code> synopsis</a></dt> </dl> </dd> <dt><a href="#examples">Example(s)</a></dt> </dl> </dd> </dl> <hr> <h2><a name="introduction"></a>Introduction</h2> <p><code><boost/python/has_back_reference.hpp></code> defines the predicate metafunction <code>has_back_reference<></code>, which can be specialized by the user to indicate that a wrapped class instance holds a <code>PyObject*</code> corresponding to a Python object.</p> <h2><a name="classes"></a>Classes</h2> <h3><a name="has_back_reference-spec"></a>Class template <code>has_back_reference</code></h3> <p>A unary metafunction whose <code>value</code> is true iff its argument is a <code>pointer_wrapper<></code>.</p> <h4><a name="has_back_reference-spec-synopsis"></a>Class template <code>has_back_reference</code> synopsis</h4> <pre> namespace boost { namespace python { template<class WrappedClass> class has_back_reference { typedef mpl::false_ type; }; }} </pre> <p>A "<a href="../../../mpl/doc/refmanual/metafunction.html"> metafunction</a>" that is inspected by Boost.Python to determine how wrapped classes can be constructed.</p> <dl class="traits-semantics"> <dt><code>type::value</code> is an integral constant convertible to bool of unspecified type.</dt> <dt>Specializations may substitute a <code>true</code>-valued integral constant wrapper for <code>type</code> iff for each invocation of <code>class_<WrappedClass>::def(init<</code> <i>type-sequence...</i><code>>())</code> and the implicitly wrapped copy constructor (unless it is <a href="class.html#class_-spec"> noncopyable</a>), there exists a corresponding constructor <code>WrappedClass::WrappedClass(PyObject*, </code> <i>type-sequence...</i><code>)</code>. If such a specialization exists, the <code>WrappedClass</code> 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 <code> type </code> is to derive the specialization from <code>mpl::true_</code>. </dt> </dl> <h2><a name="examples"></a>Example</h2> <h3>C++ module definition</h3> <pre> #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) ; } </pre> The following Python session illustrates that <code>x.self()</code> returns the same Python object on which it is invoked, while <code>y.self()</code> must create a new Python object which refers to the same Y instance. <h3>Python code</h3> <pre> >>> from back_references import * >>> x = X(1) >>> x2 = x.self() >>> x2 is x <b>1</b> >>> (x.get(), x2.get()) (1, 1) >>> x.set(10) >>> (x.get(), x2.get()) (10, 10) >>> >>> >>> y = Y(2) >>> y2 = y.self() >>> y2 is y <b>0</b> >>> (y.get(), y2.get()) (2, 2) >>> y.set(20) >>> (y.get(), y2.get()) (20, 20) </pre> <p>Revised  18 July, 2004  </p> <p class="c3">© Copyright <a href= "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002 .</p> </body> </html>