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Standard library header <optional>

From cppreference.com
< cpp‎ | header

Contents

[edit] Classes

Name Description
(since C++17)
a wrapper that may or may not hold an object
(class template) [edit]
exception indicating checked access to an optional that doesn't contain a value
(class) [edit]
specializes the std::hash algorithm
(class template specialization) [edit]
(C++17)
indicator of optional type with uninitialized state
(class) [edit]

[edit] Functions

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

[edit] Synopsis

namespace std {
  // optional
  template <class T> class optional;
  // no-value state indicator
  struct nullopt_t{/*see definition*/ };
  constexpr nullopt_t nullopt(/*unspecified*/);
  // class bad_optional_access
  class bad_optional_access;
  // relational operators
  template <class T>
  constexpr bool operator==(const optional<T>&, const optional<T>&);
  template <class T>
  constexpr bool operator!=(const optional<T>&, const optional<T>&);
  template <class T>
  constexpr bool operator<(const optional<T>&, const optional<T>&);
  template <class T>
  constexpr bool operator>(const optional<T>&, const optional<T>&);
  template <class T>
  constexpr bool operator<=(const optional<T>&, const optional<T>&);
  template <class T>
  constexpr bool operator>=(const optional<T>&, const optional<T>&);
  // 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;
  // comparison with T
  template <class T> constexpr bool operator==(const optional<T>&, const T&);
  template <class T> constexpr bool operator==(const T&, const optional<T>&);
  template <class T> constexpr bool operator!=(const optional<T>&, const T&);
  template <class T> constexpr bool operator!=(const T&, const optional<T>&);
  template <class T> constexpr bool operator<(const optional<T>&, const T&);
  template <class T> constexpr bool operator<(const T&, const optional<T>&);
  template <class T> constexpr bool operator<=(const optional<T>&, const T&);
  template <class T> constexpr bool operator<=(const T&, const optional<T>&);
  template <class T> constexpr bool operator>(const optional<T>&, const T&);
  template <class T> constexpr bool operator>(const T&, const optional<T>&);
  template <class T> constexpr bool operator>=(const optional<T>&, const T&);
  template <class T> constexpr bool operator>=(const T&, const optional<T>&);
  // specialized algorithms
  template <class T> void swap(optional<T>&, optional<T>&) noexcept(/*see definition*/);
  template <class T> constexpr optional</*see definition*/> 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>>;
}

[edit] Class template std::optional

template <class T> class optional {
public:
  using value_type = T;
  // constructors
  constexpr optional() noexcept;
  constexpr optional(nullopt_t) noexcept;
  optional(const optional&);
  optional(optional&&) noexcept(/*see definition*/ );
  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> /*conditionally-explicit*/ constexpr optional(U&&);
  template <class U> /*conditionally-explicit*/ optional(const optional<U>&);
  template <class U> /*conditionally-explicit*/ optional(optional<U> &&);
  // destructor
  ~optional();
  // assignment
  optional& operator=(nullopt_t) noexcept;
  optional& operator=(const optional&);
  optional& operator=(optional&&) noexcept(see below );
  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> void emplace(Args&&...);
  template <class U, class... Args>
  void emplace(initializer_list<U>, Args&&...);
  // swap
  void swap(optional&) noexcept(see below );
  // 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
};