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Showing content from http://en.cppreference.com/w/cpp/language/../types/is_rvalue_reference.html below:

std::is_rvalue_reference - cppreference.com

template< class T >
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< class T >
constexpr bool is_rvalue_reference_v = is_rvalue_reference<T>::value;

#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

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