Standard library header <optional> (C++17)

 

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Standard library headers

Language support <cfloat> <climits> <compare>(C++20) <contracts>(C++26) <coroutine>(C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint>(C++11) <cstdlib> <exception> <initializer_list>(C++11)    <limits> <new> <source_location>(C++20) <stdfloat>(C++23) <typeinfo> <version>(C++20) Concepts <concepts>(C++20) Diagnostics <cassert> <cerrno> <debugging>(C++26) <stacktrace>(C++23) <stdexcept> <system_error>(C++11) Memory management <memory> <memory_resource>(C++17) <scoped_allocator>(C++11) Metaprogramming <type_traits>(C++11) <ratio>(C++11)

General utilities <any>(C++17) <bitset> <bit>(C++20) <charconv>(C++17) <expected>(C++23) <format>(C++20) <functional> <optional>(C++17) <stdbit.h>(C++26) <tuple>(C++11) <typeindex>(C++11) <utility> <variant>(C++17) Containers <array>(C++11) <deque> <flat_map>(C++23) <flat_set>(C++23) <forward_list>(C++11) <hive>(C++26) <inplace_vector>(C++26)    <list> <map> <mdspan>(C++23) <queue> <set> <span>(C++20) <stack> <unordered_map>(C++11) <unordered_set>(C++11) <vector> Iterators <iterator> Ranges <generator>(C++23) <ranges>(C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar>(C++11) <cwchar> <cwctype> <string_view>(C++17) <string> Text processing <clocale> <codecvt>(C++11/17/26*) <locale> <regex>(C++11) <text_encoding>(C++26)    Numerics <cfenv>(C++11) <cmath> <complex> <linalg>(C++26) <numbers>(C++20) <random>(C++11) <simd>(C++26) <stdckdint.h>(C++26) <valarray> Time <chrono>(C++11) <ctime> C compatibility <ccomplex>(C++11/17/20*) <ciso646>(until C++20) <cstdalign>(C++11/17/20*) <cstdbool>(C++11/17/20*) <ctgmath>(C++11/17/20*)

Input/output <cinttypes>(C++11) <cstdio> <filesystem>(C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print>(C++23) <spanstream>(C++23) <sstream> <streambuf> <strstream>(C++98/26*) <syncstream>(C++20) Concurrency support <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 support <execution>(C++17)

 

This header is part of the general utility library.

Includes
<compare> (C++20)	Three-way comparison operator support
Classes
optional (C++17)	a wrapper that may or may not hold an object (class template)
bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)
std::hash<std::optional> (C++17)	hash support for std::optional (class template specialization)
nullopt_t (C++17)	indicator of an std::optional that does not contain a value (class)
Forward declarations
Defined in header <functional>
hash (C++11)	hash function object (class template)
Constants
nullopt (C++17)	an object of type nullopt_t (constant)
Functions
Comparison
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)	compares optional objects (function template)
Specialized algorithms
std::swap(std::optional) (C++17)	specializes the std::swap algorithm (function template)
make_optional (C++17)	creates an optional object (function template)

Synopsis

#include <compare>   namespace std {// class template optionaltemplate<class T>class optional;   template<class T>constexprboolranges::enable_view<optional<T>>=true;template<class T>constexprauto 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 indicatorstruct nullopt_t{/* see description */};inlineconstexpr nullopt_t nullopt(/* unspecified */);   // class bad_optional_accessclass bad_optional_access;   // relational operatorstemplate<class T, class U>constexprbool operator==(const optional<T>&, const optional<U>&);template<class T, class U>constexprbool operator!=(const optional<T>&, const optional<U>&);template<class T, class U>constexprbool operator<(const optional<T>&, const optional<U>&);template<class T, class U>constexprbool operator>(const optional<T>&, const optional<U>&);template<class T, class U>constexprbool operator<=(const optional<T>&, const optional<U>&);template<class T, class U>constexprbool 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 nullopttemplate<class T>constexprbool operator==(const optional<T>&, nullopt_t)noexcept;template<class T>constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t)noexcept;   // comparison with Ttemplate<class T, class U>constexprbool operator==(const optional<T>&, const U&);template<class T, class U>constexprbool operator==(const T&, const optional<U>&);template<class T, class U>constexprbool operator!=(const optional<T>&, const U&);template<class T, class U>constexprbool operator!=(const T&, const optional<U>&);template<class T, class U>constexprbool operator<(const optional<T>&, const U&);template<class T, class U>constexprbool operator<(const T&, const optional<U>&);template<class T, class U>constexprbool operator>(const optional<T>&, const U&);template<class T, class U>constexprbool operator>(const T&, const optional<U>&);template<class T, class U>constexprbool operator<=(const optional<T>&, const U&);template<class T, class U>constexprbool operator<=(const T&, const optional<U>&);template<class T, class U>constexprbool operator>=(const optional<T>&, const U&);template<class T, class U>constexprbool 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 algorithmstemplate<class T>constexprvoid 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 supporttemplate<class T>struct hash;template<class T>struct hash<optional<T>>;}

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 */;   // constructorsconstexpr optional()noexcept;constexpr optional(nullopt_t)noexcept;constexpr optional(const optional&);constexpr optional(optional&&)noexcept(/* see description */);template<class... Args>constexprexplicit optional(in_place_t, Args&&...);template<class U, class... Args>constexprexplicit optional(in_place_t, initializer_list<U>, Args&&...);template<class U = remove_cv_t<T>>constexprexplicit(/* see description */) optional(U&&);template<class U>constexprexplicit(/* see description */) optional(const optional<U>&);template<class U>constexprexplicit(/* see description */) optional(optional<U>&&);   // destructorconstexpr ~optional();   // assignmentconstexpr 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&&...);   // swapconstexprvoid swap(optional&)noexcept(/* see description */);   // iterator supportconstexpr iterator begin()noexcept;constexpr const_iterator begin()constnoexcept;constexpr iterator end()noexcept;constexpr const_iterator end()constnoexcept;   // observersconstexprconst T* operator->()constnoexcept;constexpr T* operator->()noexcept;constexprconst T& operator*()const&noexcept;constexpr T& operator*()&noexcept;constexpr T&& operator*()&&noexcept;constexprconst T&& operator*()const&&noexcept;constexprexplicit operator bool()constnoexcept;constexprbool has_value()constnoexcept;constexprconst T& value()const&;constexpr T& value()&;constexpr T&& value()&&;constexprconst T&& value()const&&;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 operationstemplate<class F>constexprauto and_then(F&& f)&;template<class F>constexprauto and_then(F&& f)&&;template<class F>constexprauto and_then(F&& f)const&;template<class F>constexprauto and_then(F&& f)const&&;template<class F>constexprauto transform(F&& f)&;template<class F>constexprauto transform(F&& f)&&;template<class F>constexprauto transform(F&& f)const&;template<class F>constexprauto transform(F&& f)const&&;template<class F>constexpr optional or_else(F&& f)&&;template<class F>constexpr optional or_else(F&& f)const&;   // modifiersconstexprvoid reset()noexcept;   private: T *val;// exposition only};   template<class T> optional(T)-> optional<T>;}