23 General utilities library [utilities]

23.6 Optional objects [optional]

23.6.2 Header <optional> synopsis [optional.syn]

namespace std {
  // [optional.optional], class template optional
  template<class T>
    class optional;

  // [optional.nullopt], no-value state indicator
  struct nullopt_t{see below};
  inline constexpr nullopt_t nullopt(unspecified);

  // [optional.bad.access], class bad_­optional_­access
  class bad_optional_access;

  // [optional.relops], 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>&);

  // [optional.nullops], comparison with nullopt
  template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
  template<class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept;
  template<class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;
  template<class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;
  template<class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept;
  template<class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept;
  template<class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept;

  // [optional.comp_with_t], 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>&);

  // [optional.specalg], specialized algorithms
  template<class T>
    void swap(optional<T>&, optional<T>&) noexcept(see below);

  template<class T>
    constexpr optional<see below> 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);

  // [optional.hash], hash support
  template<class T> struct hash;
  template<class T> struct hash<optional<T>>;
}
A program that necessitates the instantiation of template optional for a reference type, or for possibly cv-qualified types in_­place_­t or nullopt_­t is ill-formed.