std::add_lvalue_reference,std::add_rvalue_reference

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Metaprogramming library

Type traits

Type categories

is_void

(C++11)

is_null_pointer

(C++11)(DR*)

is_array (C++11)
is_pointer (C++11)
is_enum (C++11)
is_union (C++11)
is_class (C++11)
is_function (C++11)
is_reference (C++11)

is_object (C++11)
is_scalar (C++11)
is_compound (C++11)
is_integral (C++11)
is_floating_point (C++11)
is_fundamental (C++11)
is_arithmetic (C++11)

is_lvalue_reference

(C++11)

is_rvalue_reference

(C++11)

is_member_pointer

(C++11)

is_member_object_pointer

(C++11)

is_member_function_pointer

(C++11)

Type properties

(C++11)

(C++11)

(C++11)

(C++11)

(C++14)

(C++11)

(C++17)

(C++23)

(C++11)(deprecated in C++26)

is_trivially_copyable

(C++11)

is_standard_layout

(C++11)

is_literal_type

(C++11)(until C++20*)

is_pod

(C++11)(deprecated in C++20)

(C++11)

(C++11)

(C++20)

(C++20)

(C++23)

has_unique_object_representations

(C++17)

Type trait constants

integral_constantbool_constanttrue_typefalse_type

(C++11)(C++17)(C++11)(C++11)

Metafunctions

conjunction

(C++17)

disjunction

(C++17)

negation

(C++17)

Supported operations

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)
is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)

is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)
is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11)
has_virtual_destructor (C++11)
is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17)

Relationships and property queries

is_same (C++11)
is_convertibleis_nothrow_convertible (C++11)(C++20)
is_layout_compatible (C++20)
is_pointer_interconvertible_base_of (C++20)
is_pointer_interconvertible_with_class (C++20)
is_corresponding_member (C++20)
reference_constructs_from_temporary (C++23)
reference_converts_from_temporary (C++23)

is_base_of (C++11)
is_virtual_base_of (C++26)
alignment_of (C++11)
rank (C++11)
extent (C++11)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17)

Type modifications

remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)
add_cvadd_constadd_volatile (C++11)(C++11)(C++11)
make_signed (C++11)
make_unsigned (C++11)

remove_reference (C++11)
add_lvalue_referenceadd_rvalue_reference (C++11)(C++11)
remove_pointer (C++11)
add_pointer (C++11)
remove_extent (C++11)
remove_all_extents (C++11)

Type transformations

aligned_storage (C++11)(deprecated in C++23)
aligned_union (C++11)(deprecated in C++23)
decay (C++11)
remove_cvref (C++20)
result_ofinvoke_result (C++11)(until C++20*)(C++17)

conditional (C++11)
common_type (C++11)
common_reference (C++20)
underlying_type (C++11)
type_identity (C++20)
enable_if (C++11)
void_t (C++17)

Compile-time rational arithmetic

Compile-time integer sequences

integer_sequence

(C++14)

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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 ofT.

Type trait	The type referred by the nested type`type`
Type trait	`T` is areferenceable type	`T` is not a referenceable type
(1)	`T&`^[1]	`T`
(2)	`T&&`^[2]	`T`

↑This rule reflects the semantics ofreference collapsing.
↑This rule reflects the semantics ofreference collapsing. Note thatstd::add_rvalue_reference<T&>::type isT&, 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.

[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 usingT& orT&& is thatT can be a non-referenceable type. For example,std::add_lvalue_reference<void>::type isvoid, whilevoid& 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 afunction type withcv orref	the type produced is`T` in this case

[edit]See also

is_reference (C++11)	checks if a type is either anlvalue reference orrvalue reference (class template)[edit]
remove_reference (C++11)	removes a reference from the given type (class template)[edit]
remove_cvref (C++20)	combinesstd::remove_cv andstd::remove_reference (class template)[edit]