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

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C++
Standard library headers

This header is part of the general utility library.

Contents

Includes

(C++20)
Three-way comparison operator support

Classes

(C++17)
a wrapper that may or may not hold an object
(class template)
(C++17)
exception indicating checked access to an optional that doesn't contain a value
(class)
(C++17)
hash support for std::optional
(class template specialization)
(C++17)
indicator of an std::optional that does not contain a value
(class)
Forward declarations
Defined in header <functional>
(C++11)
hash function object
(class template)

Constants

(C++17)
an object of type nullopt_t
(constant)

Functions

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

Synopsis 

// mostly freestanding
#include <compare>
namespace std {
  // class template optional
  template<class T> class optional;                             // partially freestanding
  template<class T> constexpr bool ranges::enable_view<optional<T>> = true;
  template<class T> constexpr auto format_kind<optional<T>> = range_format::disabled;
  template<class T>
  concept /*is-derived-from-optional*/ = requires(const T& t) { // exposition-only
    []<class U>(const optional<U>&) {}(t);
  };
  // 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, class U>
    requires(!/*is-derived-from-optional*/<U>) && 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<decay_t<T>> 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;
    using iterator = /* implementation-defined */;
    using const_iterator = /* implementation-defined */;
    // 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 = remove_cv_t<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 = remove_cv_t<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 */);
    // iterator support
    constexpr iterator begin() noexcept;
    constexpr const_iterator begin() const noexcept;
    constexpr iterator end() noexcept;
    constexpr const_iterator end() const noexcept;
    // 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&;   // freestanding-deleted
    constexpr T& value() &;              // freestanding-deleted
    constexpr T&& value() &&;            // freestanding-deleted
    constexpr const T&& value() const&&; // freestanding-deleted
    template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const&;
    template<class U = remove_cv_t<T>> 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>;
}

Class template std::bad_optional_access 

namespace std {
  class bad_optional_access : public exception
  {
  public:
    // for the specification of the special member functions
    constexpr const char* what() const noexcept override;
  };
}