W3cubDocs

/C++

std::is_rvalue_reference

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.

The behavior of a program that adds specializations for std::is_rvalue_reference or std::is_rvalue_reference_v is undefined.

Template parameters

a type to check

Helper variable template

template< class T >
inline 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

#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

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