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)