Template:cpp/synopsis/optional
From cppreference.com
#include <compare> namespace std { // class template optional template<class T> class optional; // 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, 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</* see description */> 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>>; }
Class template std::optional
namespace std { template<class T> class optional { public: using value_type = T; // 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 = 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 = 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 */); // 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&; 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&&) &&; // 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>; }