std::chrono::zoned_time 的推导指引

来自cppreference.com


 
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std::chrono::zoned_time
 
zoned_time() -> zoned_time<std::chrono::seconds>;
(1) (C++20 起) 
template< class Duration >
zoned_time( std::chrono::sys_time<Duration> )
    -> zoned_time<std::common_type_t<Duration, std::chrono::seconds>>;
(2) (C++20 起) 
template< class TimeZonePtrOrName >
zoned_time( TimeZonePtrOrName&& ) -> zoned_time<std::chrono::seconds, /* 见下文 */>;
(3) (C++20 起) 
template< class TimeZonePtrOrName, class Duration >
zoned_time( TimeZonePtrOrName&&, std::chrono::sys_time<Duration> )
    -> zoned_time<std::common_type_t<Duration, std::chrono::seconds>, /* 见下文 */>;
(4) (C++20 起) 
template< class TimeZonePtrOrName, class Duration >
zoned_time( TimeZonePtrOrName&&, std::chrono::local_time<Duration>,
            std::chrono::choose = std::chrono::choose::earliest )
    -> zoned_time<std::common_type_t<Duration, std::chrono::seconds>, /* 见下文 */>;
(5) (C++20 起) 
template< class TimeZonePtrOrName, class Duration, class TimeZonePtr2 >
zoned_time( TimeZonePtrOrName&&, std::chrono::zoned_time<Duration, TimeZonePtr2>,
            std::chrono::choose = std::chrono::choose::earliest )
    -> zoned_time<std::common_type_t<Duration, std::chrono::seconds>, /* 见下文 */>;
(6) (C++20 起)

为 zoned_time 提供六个推导指引。它们把 Duration 正规化为最小分辨率为 std::chrono::seconds，并提供用对可转换成 std::string_view 的类型的指定时区名的正确处理。

3-6) 若 std::is_convertible_v<TimeZonePtrOrName, std::string_view> 为 true，则推导的第二模板实参为 const std::chrono::time_zone*。否则为 std::remove_cvref_t<TimeZonePtrOrName>。