std::is_reference

 

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integer_sequence (C++14)

 

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.

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 > 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

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