Std::add_lvalue_reference,std::add_rvalue_reference (3) Linux Manual Page

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 an object type or a function type that has no cv- or ref- qualifier, 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 an object type or a function type that has no cv- or ref- qualifier, provides a member typedef type which is T&&, otherwise type is T.

Member types

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

Helper types

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

Possible implementation

namespace detail {

template <class T>
struct type_identity { using type = T; }; // or use std::type_identity (since C++20)

  template <class T>
  auto try_add_lvalue_reference(int) -> type_identity<T&>;
  template <class T>
  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;
    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';
}

Output:

  false
  true
  true

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG_2101 C++11 These transformation traits were required Produce cv-/ref-qualified function types themselves.
to produce reference to cv-/ref-qualified function types.

std::add_lvalue_reference,std::add_rvalue_reference (3) Linux Manual Page