std::add_lvalue_reference, std::add_rvalue_reference

 

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is_lvalue_reference (C++11)
is_rvalue_reference (C++11)
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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)
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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) aligned_union (C++11) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17)	conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(until C++20)(C++17) type_identity (C++20)

 

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 an object type or a function type that has no cv- or ref- qualifier (since C++17), 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 an object type or a function type that has no cv- or ref- qualifier (since C++17), provides a member typedef type which is T&&, otherwise type is T.

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 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&&

Example

Run this code

#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

See also

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