std::add_lvalue_reference, std::add_rvalue_reference

 

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Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

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&	T
(2)	T&&

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Nested types

Name	Definition
type	determined as above

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

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.

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

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() {}

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

See also

is_reference (C++11)	checks if a type is either an lvalue reference or rvalue reference (class template)
remove_reference (C++11)	removes a reference from the given type (class template)
remove_cvref (C++20)	combines std::remove_cv and std::remove_reference (class template)