std::is_rvalue_reference(3)

NAME
std::is_rvalue_reference - std::is_rvalue_reference

Synopsis
   Defined in header <type_traits>
   template< class T > (since C++11)
   struct is_rvalue_reference;
   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 >
   inline constexpr bool is_rvalue_reference_v = (since C++17)
   is_rvalue_reference<T>::value; Inherited from std::integral_constant

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

Member functions
   operator bool converts the object to bool, returns value
                 (public member function)
   operator() returns value
   (C++14) (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 <type_traits>
 #include <iostream>
 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
   is_lvalue_reference checks if a type is an lvalue reference
   (C++11) (class template)
   is_reference checks if a type is either an lvalue reference or rvalue
   (C++11) reference
                       (class template)