# std::add_lvalue_reference, std::add_rvalue_reference

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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

Run this code

    #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.

### See also

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)

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