22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.6 Class template reference_wrapper [refwrap]

22.10.6.1 General [refwrap.general]

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namespace std { template<class T> class reference_wrapper { public: // types using type = T; // [refwrap.const], constructors template<class U> constexpr reference_wrapper(U&&) noexcept(see below); constexpr reference_wrapper(const reference_wrapper& x) noexcept; // [refwrap.assign], assignment constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept; // [refwrap.access], access constexpr operator T& () const noexcept; constexpr T& get() const noexcept; // [refwrap.invoke], invocation template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>); // [refwrap.comparisons], comparisons 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>; }
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reference_wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T.
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reference_wrapper<T> is a trivially copyable type ([basic.types.general]).
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The template parameter T of reference_wrapper may 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]

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template<class U> constexpr reference_wrapper(U&& u) noexcept(see below);
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Let FUN denote the exposition-only functions void FUN(T&) noexcept; void FUN(T&&) = delete;
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Constraints: The expression FUN(declval<U>()) is well-formed and is_same_v<remove_cvref_t<U>, reference_wrapper> is false.
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Effects: Creates a variable r as if by T& r = std​::​forward<U>(u), then constructs a reference_wrapper object that stores a reference to r.
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Remarks: The exception specification is equivalent to noexcept(FUN(declval<U>())).
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constexpr reference_wrapper(const reference_wrapper& x) noexcept;
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Effects: Constructs a reference_wrapper object that stores a reference to x.get().

22.10.6.3 Assignment [refwrap.assign]

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constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;
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Postconditions: *this stores a reference to x.get().

22.10.6.4 Access [refwrap.access]

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constexpr operator T& () const noexcept;
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Returns: The stored reference.
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constexpr T& get() const noexcept;
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Returns: The stored reference.

22.10.6.5 Invocation [refwrap.invoke]

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template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);
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Mandates: T is a complete type.
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Returns: INVOKE(get(), std​::​forward<ArgTypes>(args)...).
([func.require])

22.10.6.6 Comparisons [refwrap.comparisons]

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friend constexpr bool operator==(reference_wrapper x, reference_wrapper y);
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Constraints: The expression x.get() == y.get() is well-formed and its result is convertible to bool.
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Returns: x.get() == y.get().
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friend constexpr bool operator==(reference_wrapper x, const T& y);
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Constraints: The expression x.get() == y is well-formed and its result is convertible to bool.
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Returns: x.get() == y.
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friend constexpr bool operator==(reference_wrapper x, reference_wrapper<const T> y);
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Constraints: is_const_v<T> is false and the expression x.get() == y.get() is well-formed and its result is convertible to bool.
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Returns: x.get() == y.get().
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friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper y);
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Constraints: The expression synth-three-way(x.get(), y.get()) is well-formed.
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Returns: synth-three-way(x.get(), y.get()).
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friend constexpr auto operator<=>(reference_wrapper x, const T& y);
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Constraints: The expression synth-three-way(x.get(), y) is well-formed.
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Returns: synth-three-way(x.get(), y).
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friend constexpr auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);
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Constraints: is_const_v<T> is false.
The expression synth-three-way(x.get(), y.get()) is well-formed.
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Returns: synth-three-way(x.get(), y.get()).

22.10.6.7 Helper functions [refwrap.helpers]

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The template parameter T of the following ref and cref function templates may be an incomplete type.
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template<class T> constexpr reference_wrapper<T> ref(T& t) noexcept;
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Returns: reference_wrapper<T>(t).
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template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;
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Returns: t.
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template<class T> constexpr reference_wrapper<const T> cref(const T& t) noexcept;
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Returns: reference_wrapper<const T>(t).
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template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;
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Returns: t.

22.10.6.8 common_reference related specializations [refwrap.common.ref]

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namespace std { template<class T> constexpr bool is-ref-wrapper = false; // exposition only 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 = // exposition only 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>> && !ref-wrap-common-reference-exists-with<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>> && !ref-wrap-common-reference-exists-with<T, R, TQual<T>, RQual<R>>) struct basic_common_reference<T, R, TQual, RQual> { using type = common_reference_t<typename R::type&, TQual<T>>; }; }