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.