29 Time library [time]

29.7 Clocks [time.clock]

29.7.6 Type file_clock [time.clock.file]

29.7.6.1 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 1: 
The type that file_clock denotes can be in a different namespace than std​::​chrono, such as std​::​filesystem.
— end note]

29.7.6.2 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.

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

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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(os.getloc(), STATICALLY-WIDEN<charT>("{:L%F %T}"), t);
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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.