Defined in header <type_traits> | ||
---|---|---|
template< class T > struct add_lvalue_reference; | (1) | (since C++11) |
template< class T > struct add_rvalue_reference; | (2) | (since C++11) |
Creates a lvalue or rvalue reference type of T
.
T
is an object type or a function type that has no cv- or ref- qualifier, provides a member typedef type
which is T&
. If T
is an rvalue reference to some type U
, then type
is U&
. Otherwise, type
is T
.T
is an object type or a function type that has no cv- or ref- qualifier, provides a member typedef type
which is T&&
, otherwise type
is T
.Name | Definition |
---|---|
type | reference to T , or T if not allowed |
template< class T > using add_lvalue_reference_t = typename add_lvalue_reference<T>::type; | (since C++14) | |
template< class T > using add_rvalue_reference_t = typename add_rvalue_reference<T>::type; | (since C++14) |
These type transformations honor reference collapse rules:
std::add_lvalue_reference<T&>::type is T& std::add_lvalue_reference<T&&>::type is T& std::add_rvalue_reference<T&>::type is T& std::add_rvalue_reference<T&&>::type is T&&
namespace detail { template <class T> struct type_identity { using type = T; }; // or use std::type_identity (since C++20) template <class T> auto try_add_lvalue_reference(int) -> type_identity<T&>; template <class T> auto try_add_lvalue_reference(...) -> type_identity<T>; template <class T> auto try_add_rvalue_reference(int) -> type_identity<T&&>; template <class T> auto try_add_rvalue_reference(...) -> type_identity<T>; } // namespace detail template <class T> struct add_lvalue_reference : decltype(detail::try_add_lvalue_reference<T>(0)) {}; template <class T> struct add_rvalue_reference : decltype(detail::try_add_rvalue_reference<T>(0)) {}; |
#include <iostream> #include <type_traits> int main() { using nonref = int; using lref = typename std::add_lvalue_reference<nonref>::type; using rref = typename std::add_rvalue_reference<nonref>::type; std::cout << std::boolalpha; std::cout << std::is_lvalue_reference<nonref>::value << '\n'; std::cout << std::is_lvalue_reference<lref>::value << '\n'; std::cout << std::is_rvalue_reference<rref>::value << '\n'; }
Output:
false true true
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 2101 | C++11 | These transformation traits were required to produce reference to cv-/ref-qualified function types. | Produce cv-/ref-qualified function types themselves. |
(C++11) | checks if a type is either a lvalue reference or rvalue reference (class template) |
(C++11) | removes a reference from the given type (class template) |
(C++20) | combines std::remove_cv and std::remove_reference (class template) |
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