[optional.syn]

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// mostly freestanding #include <compare> // see [compare.syn] namespace std { // [optional.optional], class template optional template<class T> class optional; // partially freestanding template<class T> concept is-derived-from-optional = requires(const T& t) { // exposition only []<class U>(const optional&){ }(t); }; // [optional.nullopt], no-value state indicator struct nullopt_t{see below}; inline constexpr nullopt_t nullopt(unspecified); // [optional.bad.access], class bad_optional_access class bad_optional_access; // [optional.relops], relational operators template<class T, class U> constexpr bool operator==(const optional<T>&, const optional&); template<class T, class U> constexpr bool operator!=(const optional<T>&, const optional&); template<class T, class U> constexpr bool operator<(const optional<T>&, const optional&); template<class T, class U> constexpr bool operator>(const optional<T>&, const optional&); template<class T, class U> constexpr bool operator<=(const optional<T>&, const optional&); template<class T, class U> constexpr bool operator>=(const optional<T>&, const optional&); template<class T, three_way_comparable_with<T> U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const optional&); // [optional.nullops], comparison with nullopt template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept; template<class T> constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept; // [optional.comp.with.t], comparison with T template<class T, class U> constexpr bool operator==(const optional<T>&, const U&); template<class T, class U> constexpr bool operator==(const T&, const optional&); template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator!=(const T&, const optional&); template<class T, class U> constexpr bool operator<(const optional<T>&, const U&); template<class T, class U> constexpr bool operator<(const T&, const optional&); template<class T, class U> constexpr bool operator>(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>(const T&, const optional&); template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator<=(const T&, const optional&); template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>=(const T&, const optional&); template<class T, class U> requires (!is-derived-from-optional) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); // [optional.specalg], specialized algorithms template<class T> constexpr void swap(optional<T>&, optional<T>&) noexcept(see below); template<class T> constexpr optional<see below> make_optional(T&&); template<class T, class... Args> constexpr optional<T> make_optional(Args&&... args); template<class T, class U, class... Args> constexpr optional<T> make_optional(initializer_list il, Args&&... args); // [optional.hash], hash support template<class T> struct hash; template<class T> struct hash<optional<T>>; }

22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.2 Header <optional> synopsis [optional.syn]