optional
标准库标头<optional>
班
| 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) |
功能
比较
*。
运算符==运算符%21=运算符<操作符<=运算符>操作符>=%28C++17%29比较可选对象%28函数模板%29
专用算法
调换%28std：：可选的%29%28C++17%29专门使用std：：swp算法%28函数%29
制造[医]可选%28C++17%29创建可选对象%28函数模板%29
简介
二次
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>>;
}
二次
类模板std::optional
二次
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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