Standard library header <optional> (C++17)
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Standard library headers
<compare> (C++20)
<contracts> (C++26)
<coroutine> (C++20)
<cstdint> (C++11)
<source_location> (C++20)
<stdfloat> (C++23)
<version> (C++20)
<concepts> (C++20)
<debugging> (C++26)
<stacktrace> (C++23)
<system_error> (C++11)
<memory_resource> (C++17)
<scoped_allocator> (C++11)
<type_traits> (C++11)
<ratio> (C++11)
<any> (C++17)
<bit> (C++20)
<charconv> (C++17)
<expected> (C++23)
<format> (C++20)
<optional> (C++17)
<stdbit.h> (C++26)
<tuple> (C++11)
<typeindex> (C++11)
<variant> (C++17)
<array> (C++11)
<flat_map> (C++23)
<flat_set> (C++23)
<forward_list> (C++11)
<hive> (C++26)
<mdspan> (C++23)
<span> (C++20)
<unordered_map> (C++11)
<unordered_set> (C++11)
<generator> (C++23)
<ranges> (C++20)
<cuchar> (C++11)
<string_view> (C++17)
<codecvt> (C++11/17/26*)
<regex> (C++11)
<cfenv> (C++11)
<linalg> (C++26)
<numbers> (C++20)
<random> (C++11)
<simd> (C++26)
<stdckdint.h> (C++26)
<chrono> (C++11)
<ccomplex> (C++11/17/20*)
<ciso646> (until C++20)
<cstdalign> (C++11/17/20*)
<cstdbool> (C++11/17/20*)
<ctgmath> (C++11/17/20*)
<cinttypes> (C++11)
<filesystem> (C++17)
<print> (C++23)
<spanstream> (C++23)
<strstream> (C++98/26*)
<syncstream> (C++20)
<atomic> (C++11)
<barrier> (C++20)
<condition_variable> (C++11)
<future> (C++11)
<hazard_pointer> (C++26)
<latch> (C++20)
<mutex> (C++11)
<rcu> (C++26)
<semaphore> (C++20)
<shared_mutex> (C++14)
<stdatomic.h> (C++23)
<stop_token> (C++20)
<thread> (C++11)
<execution> (C++17)
This header is part of the general utility library.
Contents
Includes
Classes
(C++17)
(class) [edit]
Forward declarations
Defined in header
<functional> Constants
Functions
Comparison
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
optional objects (function template) [edit]
Specialized algorithms
[edit] Synopsis
// mostly freestanding #include <compare> namespace std { // class template optional template<class T> class optional; // partially freestanding template<class T> constexpr bool ranges::enable_view <optional<T>> = true; template<class T> constexpr auto format_kind<optional<T>> = range_format::disabled; template<class T> concept /*is-derived-from-optional*/ = requires(const T& t) { // exposition-only []<class U>(const optional<U>&) {}(t); }; // no-value state indicator struct nullopt_t { /* see description */ }; inline constexpr nullopt_t nullopt(/* unspecified */); // class bad_optional_access class bad_optional_access; // relational operators template<class T, class U> constexpr bool operator==(const optional<T>&, const optional<U>&); template<class T, class U> constexpr bool operator!=(const optional<T>&, const optional<U>&); template<class T, class U> constexpr bool operator<(const optional<T>&, const optional<U>&); template<class T, class U> constexpr bool operator>(const optional<T>&, const optional<U>&); template<class T, class U> constexpr bool operator<=(const optional<T>&, const optional<U>&); template<class T, class U> constexpr bool operator>=(const optional<T>&, const optional<U>&); template<class T, three_way_comparable_with<T> U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const optional<U>&); // 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; // 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<U>&); 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<U>&); 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<U>&); 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<U>&); 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<U>&); 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<U>&); template<class T, class U> requires(!/*is-derived-from-optional*/<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); // specialized algorithms template<class T> constexpr void swap(optional<T>&, optional<T>&) noexcept(/* see description */); template<class T> constexpr optional<decay_t<T>> 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<U> il, Args&&... args); // hash support template<class T> struct hash; template<class T> struct hash<optional<T>>; }
[edit] Class template std::optional
namespace std { template<class T> class optional { public: using value_type = T; using iterator = /* implementation-defined */; using const_iterator = /* implementation-defined */; // constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; constexpr optional(const optional&); constexpr optional(optional&&) noexcept(/* see description */); template<class... Args> constexpr explicit optional(in_place_t, Args&&...); template<class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...); template<class U = remove_cv_t<T>> constexpr explicit(/* see description */) optional(U&&); template<class U> constexpr explicit(/* see description */) optional(const optional<U>&); template<class U> constexpr explicit(/* see description */) optional(optional<U>&&); // destructor constexpr ~optional(); // assignment constexpr optional& operator=(nullopt_t) noexcept; constexpr optional& operator=(const optional&); constexpr optional& operator=(optional&&) noexcept(/* see description */); template<class U = remove_cv_t<T>> constexpr optional& operator=(U&&); template<class U> constexpr optional& operator=(const optional<U>&); template<class U> constexpr optional& operator=(optional<U>&&); template<class... Args> constexpr T& emplace(Args&&...); template<class U, class... Args> constexpr T& emplace(initializer_list<U>, Args&&...); // swap constexpr void swap(optional&) noexcept(/* see description */); // iterator support constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; // observers constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept; constexpr const T& operator*() const& noexcept; constexpr T& operator*() & noexcept; constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const&& noexcept; constexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr const T& value() const&; // freestanding-deleted constexpr T& value() &; // freestanding-deleted constexpr T&& value() &&; // freestanding-deleted constexpr const T&& value() const&&; // freestanding-deleted template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const&; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&; // monadic operations template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const&; template<class F> constexpr auto and_then(F&& f) const&&; template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const&; template<class F> constexpr auto transform(F&& f) const&&; template<class F> constexpr optional or_else(F&& f) &&; template<class F> constexpr optional or_else(F&& f) const&; // modifiers constexpr void reset() noexcept; private: T* val; // exposition-only }; template<class T> optional(T) -> optional<T>; }
[edit] Class template std::bad_optional_access
namespace std { class bad_optional_access : public exception { public: // for the specification of the special member functions constexpr const char* what() const noexcept override; }; }