Standard library header <optional>

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Standard Library header files

Language Support
<cstddef> <limits> <climits>	<cfloat> <cstdint> <new>	<typeinfo> <exception> <initializer_list>	<csignal> <csetjmp> <cstdarg>
Diagnostics
<stdexcept>	<cassert>	<cerrno>	<system_error>
General utilities
<utility> <memory> <memory_resource> <scoped_allocator>	<tuple> <optional> <any> <variant>	<bitset> <functional> <type_traits> <ratio>	<chrono> <ctime> <typeindex> <cstdlib>
Strings
<string> <string_view>	<cctype> <cwctype>	<cwchar> <cuchar>	<cstring>
Localization
<locale>	<codecvt>	<clocale>
Containers
<array> <deque> <vector>	<forward_list> <list>	<map> <set> <queue>	<unordered_map> <unordered_set> <stack>
Iterators
<iterator>
Algorithms
<algorithm>	<execution_policy>
Numerics
<complex> <random>	<valarray> <numeric>	<cfenv> <cmath>
Input/Output
<iosfwd> <iostream> <ios>	<streambuf> <istream> <ostream>	<iomanip> <sstream> <fstream>	<filesystem> <cstdio> <cinttypes>
Regular expressions
<regex>
Thread support
<thread> <atomic>	<mutex> <shared_mutex>	<condition_variable> <future>
C compatibility
<ciso646> <cstdalign>	<cstdbool> <ccomplex>	<ctgmath>

 

Classes

Name	Description
optional (since 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)

Functions

Comparison
operator==operator!=operator<operator<=operator>operator>= (C++17)	compares optional objects (function template)
Specialized algorithms
std::swap(std::optional) (C++17)	specializes the std::swap algorithm (function)
make_optional (C++17)	creates an optional object (function template)

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>>;
}

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
};