[time.clock.tai] (original) (raw)

30 Time library [time]

30.7 Clocks [time.clock]

30.7.4 Class tai_clock [time.clock.tai]

30.7.4.1 Overview [time.clock.tai.overview]

namespace std::chrono { class tai_clock { public: using rep = a signed arithmetic type;using period = ratio<_unspecified_, _unspecified_>;using duration = chrono::duration<rep, period>;using time_point = chrono::time_point<tai_clock>;static constexpr bool is_steady = unspecified;static time_point now();template<class Duration> static utc_time<common_type_t<Duration, seconds>> to_utc(const tai_time<Duration>&) noexcept;template<class Duration> static tai_time<common_type_t<Duration, seconds>> from_utc(const utc_time<Duration>&) noexcept;};}

The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date.

That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC.

Leap seconds are not inserted into TAI.

Therefore every time a leap second is inserted into UTC, UTC shifts another second with respect to TAI.

For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).

tai_clock is not a Cpp17TrivialClockunless the implementation can guarantee that tai_clock​::​now()does not propagate an exception.

[Note 1:

noexcept(from_utc(utc_clock​::​now())) is false.

— _end note_]

30.7.4.2 Member functions [time.clock.tai.members]

Returns: from_utc(utc_clock​::​now()), or a more accurate value of tai_time.

template<class Duration> static utc_time<common_type_t<Duration, seconds>> to_utc(const tai_time<Duration>& t) noexcept;

Returns: utc_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 378691210s

[Note 1: 378691210s == sys_days{1970y/January/1} - sys_days{1958y/January/1} + 10s — _end note_]

template<class Duration> static tai_time<common_type_t<Duration, seconds>> from_utc(const utc_time<Duration>& t) noexcept;

Returns: tai_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 378691210s

[Note 2: 378691210s == sys_days{1970y/January/1} - sys_days{1958y/January/1} + 10s — _end note_]

30.7.4.3 Non-member functions [time.clock.tai.nonmembers]

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

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

[Example 1: auto st = sys_days{2000y/January/1};auto tt = clock_cast<tai_clock>(st); cout << format("{0:%F %T %Z} == {1:%F %T %Z}\n", st, tt);

Produces this output:

2000-01-01 00:00:00 UTC == 2000-01-01 00:00:32 TAI

— _end example_]

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, tai_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);

Effects: Attempts to parse the input stream isinto the tai_time tp using the format flags given in the NTCTS fmtas specified in [time.parse].

If the parse fails to decode a valid date,is.setstate(ios_base​::​failbit) is called andtp 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.