std::add_lvalue_reference,std::add_rvalue_reference (3) - Linux Manuals

std::add_lvalue_reference,std::add_rvalue_reference: std::add_lvalue_reference,std::add_rvalue_reference

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 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.