20 General utilities library [utilities]

20.6 Optional objects [optional]

20.6.3 Class template optional [optional.optional] General [optional.optional.general]

namespace std { template<class T> class optional { public: using value_type = T; // [optional.ctor], constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; constexpr optional(const optional&); constexpr optional(optional&&) noexcept(see below); 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 = T> constexpr explicit(see below) optional(U&&); template<class U> constexpr explicit(see below) optional(const optional<U>&); template<class U> constexpr explicit(see below) optional(optional<U>&&); // [optional.dtor], destructor constexpr ~optional(); // [optional.assign], assignment constexpr optional& operator=(nullopt_t) noexcept; constexpr optional& operator=(const optional&); constexpr optional& operator=(optional&&) noexcept(see below); template<class U = 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&&...); // [optional.swap], swap constexpr void swap(optional&) noexcept(see below); // [optional.observe], observers constexpr const T* operator->() const; constexpr T* operator->(); constexpr const T& operator*() const&; constexpr T& operator*() &; constexpr T&& operator*() &&; constexpr const T&& operator*() const&&; constexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr const T& value() const&; constexpr T& value() &; constexpr T&& value() &&; constexpr const T&& value() const&&; template<class U> constexpr T value_or(U&&) const&; template<class U> constexpr T value_or(U&&) &&; // [optional.mod], modifiers constexpr void reset() noexcept; private: T *val; // exposition only }; template<class T> optional(T) -> optional<T>; }
Any instance of optional<T> at any given time either contains a value or does not contain a value.
When an instance of optional<T> contains a value, it means that an object of type T, referred to as the optional object's contained value, is allocated within the storage of the optional object.
Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.
The contained value shall be allocated in a region of the optional<T> storage suitably aligned for the type T.
When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.
Member val is provided for exposition only.
When an optional<T> object contains a value, val points to the contained value.
T shall be a type other than cv in_­place_­t or cv nullopt_­t that meets the Cpp17Destructible requirements (Table 32).