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std::is_reference

Defined in header <type_traits> 
template< class T >
struct is_reference;
 (since C++11)

std::is_reference is a UnaryTypeTrait.

If T is a reference type (lvalue reference or rvalue reference), provides the member constant value equal true. For any other type, value is false.

The behavior of a program that adds specializations for std::is_reference or std::is_reference_v is undefined.

Template parameters

T - a type to check

Helper variable template

template< class T >
inline constexpr bool is_reference_v = is_reference<T>::value;
 (since C++17)

Inherited from std::integral_constant

Member constants

value
[static]
true if T is a 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_reference : std::false_type {};
template<class T> struct is_reference<T&> : std::true_type {};
template<class T> struct is_reference<T&&> : std::true_type {};

Example

#include <iostream>
#include <type_traits>
 
class A {};
 
int main()
{
#   define REF(x) << #x " ?: " << x << '\n'
    std::cout << std::boolalpha
    REF(std::is_reference_v<A>)
    REF(std::is_reference_v<A&>)
    REF(std::is_reference_v<A&&>)
    REF(std::is_reference_v<long>)
    REF(std::is_reference_v<long&>)
    REF(std::is_reference_v<long&&>)
    REF(std::is_reference_v<double*>)
    REF(std::is_reference_v<double*&>)
    REF(std::is_reference_v<double*&&>);
#   undef REF
}

Output:

std::is_reference_v<A> ?: false
std::is_reference_v<A&> ?: true
std::is_reference_v<A&&> ?: true
std::is_reference_v<long> ?: false
std::is_reference_v<long&> ?: true
std::is_reference_v<long&&> ?: true
std::is_reference_v<double*> ?: false
std::is_reference_v<double*&> ?: true
std::is_reference_v<double*&&> ?: true

See also

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

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