# 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 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 > 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&&

### 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 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 a 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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