NAME¶

std::is_reference - std::is_reference

Synopsis¶

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

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.

If the program adds specializations for std::is_reference or std::is_reference_v,
the behavior is undefined.

Template parameters¶

T - a type to check

Helper variable template

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

Inherited from std::integral_constant

Member constants¶

value true if T is a 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_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¶

// Run this code

#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¶

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