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    Standard library header <optional>

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    Revision as of 14:16, 20 April 2020 by Fruderica (talk | contribs)
     
    C++
     
    Standard library headers
     
    Template:cpp/utility/hash

    Includes

    (C++20)
    		 Three-way comparison operator support[edit]

    Classes

    (C++17)
    		 a wrapper that may or may not hold an object
    (class template) [edit]
    (C++17)
    		 exception indicating checked access to an optional that doesn't contain a value
    (class) [edit]
    (C++17)
    		 hash support for std::optional
    (class template specialization) [edit]
    (C++17)
    		 indicator of an std::optional that does not contain a value
    (class) [edit]
    Forward declarations
    Defined in header <functional>

    Constants

    (C++17)
    		 an object of type nullopt_t
    (constant) [edit]

    Functions

    Comparison
    (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
    		 compares optional objects
    (function template) [edit]
    Specialized algorithms
    (C++17)
    		 specializes the std::swap algorithm
    (function template) [edit]
    (C++17)
    		 creates an optional object
    (function template) [edit]

    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>
    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>
      explicit(/* see description */) constexpr optional(U&&);
    template<class U>
      explicit(/* see description */) optional(const optional<U>&);
    template<class U>
      explicit(/* see description */) optional(optional<U>&&);

    // destructor
    ~optional();

    // assignment
    optional& operator=(nullopt_t) noexcept;
    constexpr optional& operator=(const optional&);
    constexpr optional& operator=(optional&&) noexcept(/* see description */);
    template<class U = T> optional& operator=(U&&);
    template<class U> optional& operator=(const optional<U>&);
    template<class U> optional& operator=(optional<U>&&);
    template<class... Args> T& emplace(Args&&...);
    template<class U, class... Args> T& emplace(initializer_list<U>, Args&&...);

    // swap
    void swap(optional&) noexcept(/* see description */);

    // 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&&) &&;

    // modifiers
    void reset() noexcept;

  private:
    T *val;         // exposition only
  };

  template<class T>
    optional(T) -> optional<T>;
}