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

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< cpp‎ | header

### 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) | specializes the std::hash algorithm (class template specialization)
nullopt_t (C++17) | indicator of optional type with uninitialized state (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 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>;
    }

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