# 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`.

Type trait | The type referred by the nested type `type`
--- | ---
`T` is a referenceable type | `T` is not a referenceable type
(1) | `T&`[1] | `T`
(2) | `T&&`[2]

1. ↑ This rule reflects the semantics of reference collapsing.
2. ↑ This rule reflects the semantics of reference collapsing. Note that std::add_rvalue_reference<T&>::type is `T&`, which is not an rvalue reference type.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

### Nested types

Name | Definition
`type` | determined as above

### 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

The major difference to directly using `T&` or `T&&` is that `T` can be a non-referenceable type. For example, 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>

    using non_ref = int;
    static_assert(std::is_lvalue_reference_v<non_ref> == false);

    using l_ref = std::add_lvalue_reference_t<non_ref>;
    static_assert(std::is_lvalue_reference_v<l_ref> == true);

    using r_ref = std::add_rvalue_reference_t<non_ref>;
    static_assert(std::is_rvalue_reference_v<r_ref> == true);

    using void_ref = std::add_lvalue_reference_t<void>;
    static_assert(std::is_reference_v<void_ref> == false);

    int main() {}

### 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 | the program was ill-formed if `T` is a function type with cv or ref | the type produced is `T` in this case

### 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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