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| <boost/python/reference_existing_object.hpp></title> |
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| <h1 align="center"><a href="../index.html">Boost.Python</a></h1> |
| |
| <h2 align="center">Header |
| <boost/python/reference_existing_object.hpp></h2> |
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| <h2>Contents</h2> |
| |
| <dl class="page-index"> |
| <dt><a href="#classes">Classes</a></dt> |
| |
| <dd> |
| <dl class="page-index"> |
| <dt><a href="#reference_existing_object-spec">Class |
| <code>reference_existing_object</code></a></dt> |
| |
| <dd> |
| <dl class="page-index"> |
| <dt><a href="#reference_existing_object-spec-synopsis">Class |
| <code>reference_existing_object</code> synopsis</a></dt> |
| |
| <dt><a href= |
| "#reference_existing_object-spec-metafunctions">Class |
| <code>reference_existing_object</code> metafunctions</a></dt> |
| </dl> |
| </dd> |
| </dl> |
| </dd> |
| |
| <dt><a href="#examples">Example</a></dt> |
| </dl> |
| <hr> |
| |
| <h2><a name="classes"></a>Classes</h2> |
| |
| <h3><a name="reference_existing_object-spec"></a>Class |
| <code>reference_existing_object</code></h3> |
| |
| <p><code>reference_existing_object</code> is a model of <a href= |
| "ResultConverter.html#ResultConverterGenerator-concept">ResultConverterGenerator</a> |
| which can be used to wrap C++ functions which return a reference or |
| pointer to a C++ object. When the wrapped function is called, the value |
| referenced by its return value is not copied. A new Python object is |
| created which contains a pointer to the referent, and no attempt is made |
| to ensure that the lifetime of the referent is at least as long as that |
| of the corresponding Python object. Thus, it can be <font color= |
| "#ff0000"><b>highly dangerous</b></font> to use |
| <code>reference_existing_object</code> without additional lifetime |
| management from such models of <a href= |
| "CallPolicies.html">CallPolicies</a> as <a href= |
| "with_custodian_and_ward.html#with_custodian_and_ward-spec">with_custodian_and_ward</a>. |
| This class is used in the implementation of <a href= |
| "return_internal_reference.html#return_internal_reference-spec">return_internal_reference</a>.</p> |
| |
| <h4><a name="reference_existing_object-spec-synopsis"></a>Class |
| <code>reference_existing_object</code> synopsis</h4> |
| <pre> |
| namespace boost { namespace python |
| { |
| struct reference_existing_object |
| { |
| template <class T> struct apply; |
| }; |
| }} |
| </pre> |
| |
| <h4><a name="reference_existing_object-spec-metafunctions"></a>Class |
| <code>reference_existing_object</code> metafunctions</h4> |
| <pre> |
| template <class T> struct apply |
| </pre> |
| |
| <dl class="metafunction-semantics"> |
| <dt><b>Requires:</b> <code>T</code> is <code>U&</code> or |
| <code>U*</code>for some <code>U</code>.</dt> |
| |
| <dt><b>Returns:</b> <code>typedef <a href= |
| "to_python_indirect.html#to_python_indirect-spec">to_python_indirect</a><T,V> |
| type</code>, where <code>V</code> is a class whose |
| static <code>execute</code> function constructs an instance |
| holder containing an <i>unowned</i> |
| <code>U*</code> pointing to the referent of the wrapped function's |
| return value.</dt> |
| </dl> |
| |
| <h2><a name="examples"></a>Example</h2> |
| |
| <p>In C++:</p> |
| <pre> |
| #include <boost/python/module.hpp> |
| #include <boost/python/class.hpp> |
| #include <boost/python/reference_existing_object.hpp> |
| #include <boost/python/return_value_policy.hpp> |
| #include <utility> |
| |
| // classes to wrap |
| struct Singleton |
| { |
| Singleton() : x(0) {} |
| |
| int exchange(int n) // set x and return the old value |
| { |
| std::swap(n, x); |
| return n; |
| } |
| |
| int x; |
| }; |
| |
| Singleton& get_it() |
| { |
| static Singleton just_one; |
| return just_one; |
| } |
| |
| // Wrapper code |
| using namespace boost::python; |
| BOOST_PYTHON_MODULE(singleton) |
| { |
| def("get_it", get_it, |
| return_value_policy<reference_existing_object>()); |
| |
| class_<Singleton>("Singleton") |
| .def("exchange", &Singleton::exchange) |
| ; |
| } |
| </pre> |
| In Python: |
| <pre> |
| >>> import singleton |
| >>> s1 = singleton.get_it() |
| >>> s2 = singleton.get_it() |
| >>> id(s1) == id(s2) # s1 and s2 are not the same object |
| 0 |
| >>> s1.exchange(42) # but they reference the same C++ Singleton |
| 0 |
| >>> s2.exchange(99) |
| 42 |
| </pre> |
| |
| <p>Revised |
| <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
| 13 November, 2002 |
| <!--webbot bot="Timestamp" endspan i-checksum="39359" --> |
| </p> |
| |
| <p><i>© Copyright <a href= |
| "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p> |
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