std::add_lvalue_reference, std::add_rvalue_reference
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
.
1) If
T
is a function type that has no cv- or ref- qualifier or an object type, 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
.
2) If
T
is a function type that has no cv- or ref- qualifier or an object type, provides a member typedef
type
which is
T&&
, otherwise
type
is
T
.
The behavior of a program that adds specializations for any of the templates described on this page is undefined.
Member types
Name | Definition |
---|---|
type |
reference to T , or T if not allowed |
Helper types
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) |
Notes
These type transformations honor reference collapse rules:
-
std::add_lvalue_reference<T&>::type
isT&
-
std::add_lvalue_reference<T&&>::type
isT&
-
std::add_rvalue_reference<T&>::type
isT&
-
std::add_rvalue_reference<T&&>::type
isT&&
The major difference to directly using T&
is that std::add_lvalue_reference<void>::type
is void
, while void&
leads to a compilation error.
Possible implementation
namespace detail { template <class T> struct type_identity { using type = T; }; // or use std::type_identity (since C++20) template <class T> // Note that `cv void&` is a substitution failure auto try_add_lvalue_reference(int) -> type_identity<T&>; template <class T> // Handle T = cv void case 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)) {}; |
Example
#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; using voidref = std::add_lvalue_reference_t<void>; 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'; std::cout << std::is_reference_v<voidref> << '\n'; }
Output:
false true true false
Defect reports
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. |
See also
(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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