std::add_lvalue_reference,std::add_rvalue_reference(3)

NAME
std::add_lvalue_reference,std::add_rvalue_reference - std::add_lvalue_reference,std::add_rvalue_reference

Synopsis
   Defined in header <type_traits>
   template< class T > (1) (since C++11)
   struct add_lvalue_reference;
   template< class T > (2) (since C++11)
   struct add_rvalue_reference;
   Creates a lvalue or rvalue reference type of T.
   1) If T is a function type that has no cv- or ref- qualifier or an object type,
   provides a member typedef type which is T&. If T is an rvalue reference to some type
   U, then type is U&. Otherwise, type is T.
   2) If T is a function type that has no cv- or ref- qualifier or an object type,
   provides a member typedef type which is T&&, otherwise type is T.
   The behavior of a program that adds specializations for any of the templates
   described on this page is undefined.

Member types
   Name Definition
   type reference to T, or T if not allowed

Helper types
   template< class T >
   using add_lvalue_reference_t = typename (since C++14)
   add_lvalue_reference<T>::type;
   template< class T >
   using add_rvalue_reference_t = typename (since C++14)
   add_rvalue_reference<T>::type;

Notes
   These type transformations honor reference collapse rules:
     * std::add_lvalue_reference<T&>::type is T&
     * std::add_lvalue_reference<T&&>::type is T&
     * std::add_rvalue_reference<T&>::type is T&
     * std::add_rvalue_reference<T&&>::type is T&&
   The major difference to directly using T& is that
   std::add_lvalue_reference<void>::type is void, while void& leads to a compilation
   error.

Possible implementation
   namespace detail {
   template <class T>
   struct type_identity { using type = T; }; // or use std::type_identity (since C++20)
   template <class T> // Note that `cv void&` is a substitution failure
   auto try_add_lvalue_reference(int) -> type_identity<T&>;
   template <class T> // Handle T = cv void case
   auto try_add_lvalue_reference(...) -> type_identity<T>;
   template <class T>
   auto try_add_rvalue_reference(int) -> type_identity<T&&>;
   template <class T>
   auto try_add_rvalue_reference(...) -> type_identity<T>;
   } // namespace detail
   template <class T>
   struct add_lvalue_reference : decltype(detail::try_add_lvalue_reference<T>(0)) {};
   template <class T>
   struct add_rvalue_reference : decltype(detail::try_add_rvalue_reference<T>(0)) {};

Example
// Run this code
 #include <iostream>
 #include <type_traits>
 int main() {
    using nonref = int;
    using lref = typename std::add_lvalue_reference<nonref>::type;
    using rref = typename std::add_rvalue_reference<nonref>::type;
    using voidref = std::add_lvalue_reference_t<void>;
    std::cout << std::boolalpha;
    std::cout << std::is_lvalue_reference<nonref>::value << '\n';
    std::cout << std::is_lvalue_reference<lref>::value << '\n';
    std::cout << std::is_rvalue_reference<rref>::value << '\n';
    std::cout << std::is_reference_v<voidref> << '\n';
 }

Output:
 false
 true
 true
 false
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       These transformation traits were
   LWG 2101 C++11 required Produce cv-/ref-qualified
                       to produce reference to function types themselves.
                       cv-/ref-qualified function types.

See also
   is_reference checks if a type is either a lvalue reference or rvalue reference
   (C++11) (class template)
   remove_reference removes a reference from the given type
   (C++11) (class template)
   remove_cvref combines std::remove_cv and std::remove_reference
   (C++20) (class template)