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std::is_reference

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

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 is_reference or is_reference_v (since C++17) is undefined.

Contents

[edit] Template parameters

T - a type to check

[edit] 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>

[edit] 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 {};

[edit] 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

[edit] See also

checks if a type is a lvalue reference
(class template) [edit]
checks if a type is a rvalue reference
(class template) [edit]