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)

 

Standard Library header files

Language Support
<cstddef> <limits> <climits> <cfloat> <cstdint> <new> <typeinfo> <exception> <ciso646>	<initializer_list> <csignal> <csetjmp> <cstdalign> <cstdarg> <cstdbool> <cstdlib> <ctime>
Diagnostics
<stdexcept> <cassert>	<cerrno> <system_error>
General utilities
<utility> <tuple> <bitset> <memory> <cstdlib> <cstring> <functional>	<type_traits> <ratio> <chrono> <ctime> <optional> <scoped_allocator> <typeindex>
Strings
<string> <cctype> <cwctype> <cstring>	<cwchar> <cstdlib> <cuchar>
Localization
<locale> <codectv> <clocale>

Containers
<array> <deque> <dynarray> <forward_list> <list> <vector>	<map> <set> <unordered_map> <unordered_set> <queue> <stack>
Iterators
<iterator>
Algorithms
<algorithm>	<cstdlib>
Numerics
<cfenv> <complex> <random> <valarray>	<numeric> <cmath> <ctgmath> <cstdlib>
Input/Output
<iosfwd> <iostream> <ios> <streambuf> <istream> <ostream>	<iomanip> <sstream> <fstream> <cstdio> <cinttypes>
Regular expressions
<regex>
Thread support
<thread> <mutex> <atomic>	<shared_mutex> <condition_variable> <future>

 

This header is part of the general utility library.

Includes
<initializer_list>
Classes
optional (C++14)	types that either hold an object or don't (class template)
bad_optional_access (C++14)	exception indicating access to a optional type with uninitialized state (class)
in_place_t (C++14)	disambiguation tag type for in-place construction of optional types (class)
std::hash<std::optional>	specializes the std::hash algorithm (class template specialization)
nullopt_t (C++14)	indicator of optional type with uninitialized state (class)
Functions
operator== operator<	compares optional objects (function template)
make_optional	creates an optional object (function template)
std::swap(std::optional)	specializes the std::swap algorithm (function)

Synopsis

#include <initializer_list>
namespace std {
    // 20.6.4, optional for object types
    template <class T> class optional;
    // 20.6.5, In-place construction
    struct in_place_t{};
    constexpr in_place_t in_place{};
    // 20.6.6, Disengaged state indicator
    struct nullopt_t;
    constexpr nullopt_t nullopt( /*unspecified*/ );
    // 20.6.7, class bad_optional_access
    class bad_optional_access;
    // 20.6.8, 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>&);
    // 20.6.9, 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;
    // 20.6.10, 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>&);
    // 20.6.11, Specialized algorithms
    template <class T> void swap(optional<T>&, optional<T>&) noexcept(see below );
    template <class T> constexpr optional<T> make_optional(T&&);
    // 20.6.12, hash support
    template <class T> struct hash;
    template <class T> struct hash<optional<T>>;
} // namespace std

Class std::optional

template <class T>
class optional
{
public:
    typedef T value_type;
 
    // 20.6.4.1, constructors
    constexpr optional() noexcept;
    constexpr optional(nullopt_t) noexcept;
    optional(const optional&);
    optional(optional&&) noexcept(see below );
    constexpr optional(const T&);
    constexpr optional(T&&);
    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&&...);
 
    // 20.6.4.2, destructor
    ~optional();
 
    // 20.6.4.3, assignment
    optional& operator=(nullopt_t) noexcept;
    optional& operator=(const optional&);
    optional& operator=(optional&&) noexcept(see below );
    template <class U> optional& operator=(U&&);
    template <class... Args> void emplace(Args&&...);
    template <class U, class... Args> void emplace(initializer_list<U>, Args&&...);
 
    // 20.6.4.4, swap
    void swap(optional&) noexcept(see below );
 
    // 20.6.4.5, observers
    constexpr T const* operator->() const;
    T* operator->();
    constexpr T const& operator*() const;
    T& operator*();
    constexpr explicit operator bool() const noexcept;
    constexpr T const& value() const;
    T& value();
    template <class U> constexpr T value_or(U&&) const&;
    template <class U> T value_or(U&&) &&;
 
private:
    bool init; // exposition only
    T* val; // exposition only
};