...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
reference_existing_object
reference_existing_object
synopsisreference_existing_object
metafunctionsreference_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
synopsisnamespace boost { namespace python { struct reference_existing_object { template <class T> struct apply; }; }}
reference_existing_object
metafunctionstemplate <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(singleton) { def("get_it", get_it, return_value_policy<reference_existing_object>()); class_<Singleton>("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 13 November, 2002
© Copyright Dave Abrahams 2002.