std::add_lvalue_reference, std::add_rvalue_reference

< cpp‎ | types
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(C++11)(until C++20*)
(C++11)(deprecated in C++20)
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Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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.


[edit] Member types

Name Definition
type reference to T, or T if not allowed

[edit] 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)

[edit] Notes

These type transformations honor reference collapsing 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.

[edit] 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)) {};

[edit] 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>;
        (  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

[edit] 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.

[edit] See also

checks if a type is either an lvalue reference or rvalue reference
(class template) [edit]
removes a reference from the given type
(class template) [edit]
combines std::remove_cv and std::remove_reference
(class template) [edit]