|
+ |
+
+ Boost.Python+Header <boost/python/reference_existing_object.hpp>+ |
+
reference_existing_object
+reference_existing_object is a model of ResultConverterGenerator
+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 highly dangerous to use
+reference_existing_object without additional lifetime
+management from such models of CallPolicies as with_custodian_and_ward. This class is used in the implementation
+of return_internal_reference.
+
reference_existing_object synopsis
+namespace boost { namespace python
+{
+ struct reference_existing_object
+ {
+ template <class T> struct apply;
+ };
+}}
+
+reference_existing_object metafunctions+template <class T> struct apply ++ + +
In C++: + +
+#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_INIT(singleton)
+{
+ module m("singleton")
+ .def("get_it", get_it)
+ .add(
+ class_<Singleton>()
+ .def("exchange", &Singleton::exchange)
+ );
+}
+
+
+In Python:
+
++>>> 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 ++ +
Revised + + 14 February 2002 + +
+© Copyright Dave Abrahams + 2002. All Rights Reserved.
+ +