22 General utilities library [utilities] (original) (raw)

22.10 Function objects [function.objects]

22.10.6 Class template reference_wrapper [refwrap]

22.10.6.1 General [refwrap.general]

namespace std { template<class T> class reference_wrapper { public: using type = T;template<class U> constexpr reference_wrapper(U&&) noexcept(see below);constexpr reference_wrapper(const reference_wrapper& x) noexcept;constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;constexpr operator T& () const noexcept;constexpr T& get() const noexcept;template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);friend constexpr bool operator==(reference_wrapper, reference_wrapper);friend constexpr bool operator==(reference_wrapper, const T&);friend constexpr bool operator==(reference_wrapper, reference_wrapper<const T>);friend constexpr auto operator<=>(reference_wrapper, reference_wrapper);friend constexpr auto operator<=>(reference_wrapper, const T&);friend constexpr auto operator<=>(reference_wrapper, reference_wrapper<const T>);};template<class T> reference_wrapper(T&) -> reference_wrapper<T>;}

The template parameter T of reference_wrappermay be an incomplete type.

[Note 1:

Using the comparison operators described in [refwrap.comparisons]with T being an incomplete type can lead to an ill-formed program with no diagnostic required ([temp.point], [temp.constr.atomic]).

— _end note_]

22.10.6.2 Constructors [refwrap.const]

template<class U> constexpr reference_wrapper(U&& u) noexcept(_see below_);

Let FUN denote the exposition-only functionsvoid FUN(T&) noexcept;void FUN(T&&) = delete;

Constraints: The expression FUN(declval<U>()) is well-formed andis_same_v<remove_cvref_t<U>, reference_wrapper> is false.

Effects: Creates a variable ras if by T& r = std​::​forward<U>(u), then constructs a reference_wrapper object that stores a reference to r.

Remarks: The exception specification is equivalent tonoexcept(FUN(declval<U>())).

constexpr reference_wrapper(const reference_wrapper& x) noexcept;

Effects: Constructs a reference_wrapper object that stores a reference to x.get().

22.10.6.3 Assignment [refwrap.assign]

constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;

Postconditions: *this stores a reference to x.get().

22.10.6.4 Access [refwrap.access]

constexpr operator T& () const noexcept;

Returns: The stored reference.

constexpr T& get() const noexcept;

Returns: The stored reference.

22.10.6.5 Invocation [refwrap.invoke]

template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);

Mandates: T is a complete type.

Returns: INVOKE(get(), std​::​forward<ArgTypes>(args)...) ([func.require]).

22.10.6.6 Comparisons [refwrap.comparisons]

friend constexpr bool operator==(reference_wrapper x, reference_wrapper y);

Constraints: The expression x.get() == y.get() is well-formed and its result is convertible to bool.

Returns: x.get() == y.get().

friend constexpr bool operator==(reference_wrapper x, const T& y);

Constraints: The expression x.get() == y is well-formed and its result is convertible to bool.

friend constexpr bool operator==(reference_wrapper x, reference_wrapper<const T> y);

Constraints: is_const_v<T> is false and the expression x.get() == y.get() is well-formed and its result is convertible to bool.

Returns: x.get() == y.get().

friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper y);

Constraints: The expression synth-three-way(x.get(), y.get())is well-formed.

Returns: synth-three-way(x.get(), y.get()).

friend constexpr auto operator<=>(reference_wrapper x, const T& y);

Constraints: The expression synth-three-way(x.get(), y)is well-formed.

Returns: synth-three-way(x.get(), y).

friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);

Constraints: is_const_v<T> is false.

The expression synth-three-way(x.get(), y.get())is well-formed.

Returns: synth-three-way(x.get(), y.get()).

22.10.6.7 Helper functions [refwrap.helpers]

The template parameter T of the following ref and cref function templates may be an incomplete type.

template<class T> constexpr reference_wrapper<T> ref(T& t) noexcept;

Returns: reference_wrapper<T>(t).

template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;

template<class T> constexpr reference_wrapper<const T> cref(const T& t) noexcept;

Returns: reference_wrapper<const T>(t).

template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;

22.10.6.8 common_reference related specializations [refwrap.common.ref]

namespace std { template<class T> constexpr bool is-ref-wrapper = false; template<class T> constexpr bool is-ref-wrapper<reference_wrapper<T>> = true;template<class R, class T, class RQ, class TQ> concept ref-wrap-common-reference-exists-with = is-ref-wrapper<R> && requires { typename common_reference_t<typename R::type&, TQ>; } && convertible_to<RQ, common_reference_t<typename R::type&, TQ>>;template<class R, class T, template<class> class RQual, template<class> class TQual> requires (ref-wrap-common-reference-exists-with<R, T, RQual<R>, TQual<T>> && <T, R, TQual<T>, RQual<R>>) struct basic_common_reference<R, T, RQual, TQual> { using type = common_reference_t<typename R::type&, TQual<T>>;};template<class T, class R, template<class> class TQual, template<class> class RQual> requires (ref-wrap-common-reference-exists-with<R, T, RQual<R>, TQual<T>> && <T, R, TQual<T>, RQual<R>>) struct basic_common_reference<T, R, TQual, RQual> { using type = common_reference_t<typename R::type&, TQual<T>>;};}