Standard library header <optional>
Classes |
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(C++17) | a wrapper that may or may not hold an object (class template) |
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(C++17) | exception indicating checked access to an optional that doesn't contain a value (class) |
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(C++17) | specializes the std::hash algorithm (class template specialization) |
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(C++17) | indicator of optional type with uninitialized state (class) |
Constants |
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(C++17) | an object of type nullopt_t (constant) |
Functions |
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Comparison |
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(C++17) | compares optional objects (function template) |
Specialized algorithms |
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(C++17) | specializes the std::swap algorithm (function) |
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(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
};
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