std::is_rvalue_reference
From cppreference.com
| 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