std::add_lvalue_reference, std::add_rvalue_reference
From cppreference.com
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 an lvalue or rvalue reference type of T.
| Type trait | The type referred by the nested type type | |
|---|---|---|
T is a referenceable type | T is not a referenceable type | |
| (1) | T&[1] | T |
| (2) | T&&[2] | |
- ↑This rule reflects the semantics of reference collapsing.
- ↑This rule reflects the semantics of reference collapsing. Note that std::add_rvalue_reference<T&>::type is
T&, which is not an rvalue reference type.
If the program adds specializations for any of the templates described on this page, the behavior is undefined.
Contents |
[edit]Nested types
| Name | Definition |
type | determined as above |
[edit]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) | |
[edit]Notes
The major difference to directly using T& or T&& is that T can be a non-referenceable type. For example, std::add_lvalue_reference<void>::type is void, while void& leads to a compilation error.
[edit]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 failureauto try_add_lvalue_reference(int)-> type_identity<T&>;template<class T>// Handle T = cv void caseauto 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)){}; |
[edit]Example
Run this code
#include <type_traits> using non_ref =int; static_assert(std::is_lvalue_reference_v<non_ref>==false); using l_ref = std::add_lvalue_reference_t<non_ref>; static_assert(std::is_lvalue_reference_v<l_ref>==true); using r_ref = std::add_rvalue_reference_t<non_ref>; static_assert(std::is_rvalue_reference_v<r_ref>==true); using void_ref = std::add_lvalue_reference_t<void>; static_assert(std::is_reference_v<void_ref>==false); int main(){}
[edit]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 | the program was ill-formed if T is a function type with cv or ref | the type produced is T in this case |
[edit]See also
(C++11) | checks if a type is either an 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) |
