27 Time library [time]

27.7 Clocks [time.clock]

27.7.5 Type file_­clock [time.clock.file]

27.7.5.1 Overview [time.clock.file.overview]

namespace std::chrono {
  using file_clock = see below;
}
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file_­clock is an alias for a type meeting the Cpp17TrivialClock requirements ([time.clock.req]), and using a signed arithmetic type for file_­clock​::​rep.
file_­clock is used to create the time_­point system used for file_­time_­type ([filesystems]).
Its epoch is unspecified, and noexcept(file_­clock​::​now()) is true.
[Note
:
The type that file_­clock denotes may be in a different namespace than std​::​chrono, such as std​::​filesystem.
— end note
]

27.7.5.2 Member functions [time.clock.file.members]

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The type denoted by file_­clock provides precisely one of the following two sets of static member functions: 
template<class Duration>
  static sys_time<see below>
    to_sys(const file_time<Duration>&);
template<class Duration>
  static file_time<see below>
    from_sys(const sys_time<Duration>&);
or: 
template<class Duration>
  static utc_time<see below>
    to_utc(const file_time<Duration>&);
template<class Duration>
  static file_time<see below>
    from_utc(const utc_time<Duration>&);
These member functions shall provide time_­point conversions consistent with those specified by utc_­clock, tai_­clock, and gps_­clock.
The Duration of the resultant time_­point is computed from the Duration of the input time_­point.

27.7.5.3 Non-member functions [time.clock.file.nonmembers]

🔗
template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& t);
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Effects: Equivalent to: 
return os << format(STATICALLY-WIDEN<charT>("{:%F %T}"), t);
🔗
template<class charT, class traits, class Duration, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, file_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
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Effects: Attempts to parse the input stream is into the file_­time tp using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid date, is.setstate(ios_­base​::​failbit) is called and tp is not modified.
If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.
If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.
Additionally, the parsed offset will be subtracted from the successfully parsed timestamp prior to assigning that difference to tp.
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Returns: is.