[time.clock.file]

# 30 Time library [[time]](./#time)

## 30.7 Clocks [[time.clock]](time.clock#file)

### 30.7.6 Type file_clock [time.clock.file]

#### [30.7.6.1](#overview) Overview [[time.clock.file.overview]](time.clock.file.overview)

namespace std::chrono {using file_clock = *see below*;}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3369)

file_clock is an alias for a type
meeting the [*Cpp17TrivialClock*](time.clock.req#:Cpp17TrivialClock "30.3 Cpp17Clock requirements [time.clock.req]") requirements ([[time.clock.req]](time.clock.req "30.3 Cpp17Clock requirements")), and
using a signed arithmetic type for file_clock​::​rep[.](#overview-1.sentence-1)

file_clock is used to create the time_point system
used for file_time_type ([[filesystems]](filesystems "31.12 File systems"))[.](#overview-1.sentence-2)

Its epoch is unspecified, andnoexcept(file_clock​::​now()) is true[.](#overview-1.sentence-3)

[*Note [1](#overview-note-1)*:

The type that file_clock denotes can be
in a different namespace than std​::​chrono,
such as std​::​filesystem[.](#overview-1.sentence-4)

— *end note*]

#### [30.7.6.2](#members) Member functions [[time.clock.file.members]](time.clock.file.members)

[1](#members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3386)

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 byutc_clock, tai_clock, and gps_clock[.](#members-1.sentence-2)

The Duration of the resultant time_point is computed from the Duration of the input time_point[.](#members-1.sentence-3)

#### [30.7.6.3](#nonmembers) Non-member functions [[time.clock.file.nonmembers]](time.clock.file.nonmembers)

[🔗](#lib:operator%3c%3c,file_time)

`template<class charT, class traits, class Duration>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& t);
`

[1](#nonmembers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3423)

*Effects*: Equivalent to:return os << format(os.getloc(), *STATICALLY-WIDEN*<charT>("{:L%F %T}"), t);

[🔗](#lib:from_stream,file_time)

`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);
`

[2](#nonmembers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3441)

*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]](time.parse "30.13 Parsing")[.](#nonmembers-2.sentence-1)

If the parse fails to decode a valid date,is.setstate(ios_base​::​failbit) is called andtp is not modified[.](#nonmembers-2.sentence-2)

If %Z is used and successfully parsed,
that value will be assigned to *abbrev if abbrev is non-null[.](#nonmembers-2.sentence-3)

If %z (or a modified variant) is used and successfully parsed,
that value will be assigned to *offset if offset is non-null[.](#nonmembers-2.sentence-4)

Additionally, the parsed offset will be subtracted from
the successfully parsed timestamp prior to assigning that difference to tp[.](#nonmembers-2.sentence-5)

[3](#nonmembers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/time.tex#L3457)

*Returns*: is[.](#nonmembers-3.sentence-1)