std::add_lvalue_reference, std::add_rvalue_reference

Defined in header `<type_traits>`
`template< class T > struct add_lvalue_reference;`	(1)	(since C++11)
`template< class T > struct add_rvalue_reference;`	(2)	(since C++11)

Creates an 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 add_lvalue_reference<T>::type;`		(since C++14)
`template< class T > using add_rvalue_reference_t = typename add_rvalue_reference<T>::type;`		(since C++14)

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

#include <type_traits>
 
int main()
{
    using non_ref = int;
    using l_ref = typename std::add_lvalue_reference_t<non_ref>;
    using r_ref = typename std::add_rvalue_reference_t<non_ref>;
    using void_ref = std::add_lvalue_reference_t<void>;
 
    static_assert
        (  std::is_lvalue_reference_v<non_ref> == false
        && std::is_lvalue_reference_v<l_ref> == true
        && std::is_rvalue_reference_v<r_ref> == true
        && std::is_reference_v<void_ref> == 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
LWG 2101	C++11	These transformation traits were required to produce reference to cv-/ref-qualified function types.	Produce cv-/ref-qualified function types themselves.

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

Docs

Docs4dev

Title here