std::add_lvalue_reference, std::add_rvalue_reference

Creates an lvalue or rvalue reference type of T.

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&

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

Nested types

Helper types

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&

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>
 
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.

See also