Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The Ref library is a small library that is useful for passing references
to function templates (algorithms) that would usually take copies of their
arguments. It defines the class template boost::reference_wrapper<T>,
two functions boost::ref and boost::cref
that return instances of boost::reference_wrapper<T>,
a function boost::unwrap_ref that unwraps a boost::reference_wrapper<T> or
returns a reference to any other type of object, and the two traits classes
boost::is_reference_wrapper<T> and
boost::unwrap_reference<T>.
The purpose of boost::reference_wrapper<T> is
to contain a reference to an object of type T.
It is primarily used to "feed" references to function templates
(algorithms) that take their parameter by value.
To support this usage, boost::reference_wrapper<T>
provides an implicit conversion to T&. This usually allows the function templates
to work on references unmodified.
boost::reference_wrapper<T> is
both CopyConstructible and Assignable (ordinary references are not Assignable).
The expression boost::ref(x) returns
a boost::reference_wrapper<X>(x) where
X is the type of x. Similarly, boost::cref(x)
returns a boost::reference_wrapper<X const>(x).
The expression boost::unwrap_ref(x) returns
a boost::unwrap_reference<X>::type&
where X is the type of x.
The expression boost::is_reference_wrapper<T>::value is true
if T is a reference_wrapper,
and false otherwise.
The type-expression boost::unwrap_reference<T>::type is T::type
if T is a reference_wrapper,
T otherwise.
namespace boost { template<typename T> struct is_reference_wrapper; template<typename T> class reference_wrapper; template<typename T> struct unwrap_reference; template<typename T> reference_wrapper< T > const ref(T &); template<typename T> reference_wrapper< T const > const cref(T const &); template<typename T> void ref(T const &&); template<typename T> void cref(T const &&); template<typename T> unwrap_reference< T >::type & unwrap_ref(T &); }
ref and cref
were originally part of the Tuple library by Jaakko Järvi. They were
"promoted to boost::
status" by Peter Dimov because they are generally useful. Douglas Gregor
and Dave Abrahams contributed is_reference_wrapper
and unwrap_reference. Frank
Mori Hess and Ronald Garcia contributed boost::unwrap_ref.