22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.3 Class template optional [optional.optional]

22.5.3.1 General [optional.optional.general]

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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 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> constexpr T value_or(U&&) const &; template<class U> constexpr T value_or(U&&) &&; // [optional.monadic], 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 &; // [optional.mod], modifiers constexpr void reset() noexcept; private: T *val; // exposition only }; template<class T> optional(T) -> optional<T>; }
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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.
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.
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When an optional<T> object contains a value, member val points to the contained value.
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T shall be a type other than cv in_place_t or cv nullopt_t that meets the Cpp17Destructible requirements (Table 35).