std::is_reference(3)

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)