std:: is_rvalue_reference

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C++
Metaprogramming library
Type traits
Type categories
(C++11)
(C++11)
is_rvalue_reference
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
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(C++11)
(C++11)
(C++11)
(C++14)
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(C++11) (deprecated in C++26)
(C++11)
(C++11)
(C++11) ( until C++20* )
(C++11) (deprecated in C++20)
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Type trait constants
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(C++11)
(C++11)
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(C++11)
(C++11) ( until C++20* ) (C++17)
(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
Defined in header <type_traits>
template < class T >
struct is_rvalue_reference ;
(since C++11)

std::is_rvalue_reference is a UnaryTypeTrait .

Checks whether T is an rvalue reference type. Provides the member constant value which is equal to true , if T is an rvalue reference type. Otherwise, value is equal to false .

If the program adds specializations for std::is_rvalue_reference or std::is_rvalue_reference_v , the behavior is undefined.

Template parameters

T - a type to check

Helper variable template

template < class T >
constexpr bool is_rvalue_reference_v = is_rvalue_reference < T > :: value ;
(since C++17)

Inherited from std:: integral_constant

Member constants

value
[static]
true if T is an rvalue reference type, false otherwise
(public static member constant)

Member functions

operator bool
converts the object to bool , returns value
(public member function)
operator()
(C++14)
returns value
(public member function)

Member types

Type Definition
value_type bool
type std:: integral_constant < bool , value >

Possible implementation 

template<class T> struct is_rvalue_reference : std::false_type {};
template<class T> struct is_rvalue_reference<T&&> : std::true_type {};

Example

Run this code 
#include <iostream>
#include <type_traits>
 
class A {};
 
static_assert
(
    std::is_rvalue_reference_v<A> == false and
    std::is_rvalue_reference_v<A&> == false and
    std::is_rvalue_reference_v<A&&> != false and
    std::is_rvalue_reference_v<char> == false and
    std::is_rvalue_reference_v<char&> == false and
    std::is_rvalue_reference_v<char&&> != false
);
 
template <typename T>
void test(T&& x)
{
    static_assert(std::is_same_v<T&&, decltype(x)>);
    std::cout << "T\t" << std::is_rvalue_reference<T>::value << '\n';
    std::cout << "T&&\t" << std::is_rvalue_reference<T&&>::value << '\n';
    std::cout << "decltype(x)\t" << std::is_rvalue_reference<decltype(x)>::value << '\n';
}
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << "A\t" << std::is_rvalue_reference<A>::value << '\n';
    std::cout << "A&\t" << std::is_rvalue_reference<A&>::value << '\n';
    std::cout << "A&&\t" << std::is_rvalue_reference<A&&>::value << '\n';
    std::cout << "char\t" << std::is_rvalue_reference<char>::value << '\n';
    std::cout << "char&\t" << std::is_rvalue_reference<char&>::value << '\n';
    std::cout << "char&&\t" << std::is_rvalue_reference<char&&>::value << '\n';
 
    std::cout << "\ntest(42)\n";
    test(42);
 
    std::cout << "\ntest(x)\n";
    int x = 42;
    test(x);
}

Output: 

A	false
A&	false
A&&	true
char	false
char&	false
char&&	true
 
test(42)
T	false
T&&	true
decltype(x)	true
 
test(x)
T	false
T&&	false
decltype(x)	false

See also

(C++11)
checks if a type is an lvalue reference
(class template)
(C++11)
checks if a type is either an lvalue reference or rvalue reference
(class template)
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