From Jason Turner

[time.clock.gps]

C++17 → C++20 4.4 KB 143 lines
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C++ Weekly: Ep 397

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+ ### Class `gps_clock` <a id="time.clock.gps">[[time.clock.gps]]</a>
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+
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+ #### Overview <a id="time.clock.gps.overview">[[time.clock.gps.overview]]</a>
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+
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+ ``` cpp
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+ namespace std::chrono {
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+ class gps_clock {
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+ public:
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+ using rep = a signed arithmetic type;
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+ using period = ratio<unspecified, unspecified>;
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+ using duration = chrono::duration<rep, period>;
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+ using time_point = chrono::time_point<gps_clock>;
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+ static constexpr bool is_steady = unspecified;
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+
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+ static time_point now();
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+
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+ template<class Duration>
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+ static utc_time<common_type_t<Duration, seconds>>
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+ to_utc(const gps_time<Duration>&) noexcept;
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+ template<class Duration>
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+ static gps_time<common_type_t<Duration, seconds>>
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+ from_utc(const utc_time<Duration>&) noexcept;
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+ };
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+ }
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+ ```
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+
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+ The clock `gps_clock` measures seconds since the first Sunday of
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+ January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS.
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+ Therefore every time a leap second is inserted into UTC, UTC shifts
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+ another second with respect to GPS. Aside from the offset from
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+ `1958y/January/1` to `1980y/January/Sunday[1]`, GPS is behind TAI by 19s
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+ due to the 10s offset between 1958 and 1970 and the additional 9 leap
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+ seconds inserted between 1970 and 1980.
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+
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+ `gps_clock` is not a *Cpp17TrivialClock* unless the implementation can
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+ guarantee that `gps_clock::now()` does not propagate an exception.
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+
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+ [*Note 1*: `noexcept(from_utc(utc_clock::now()))` is
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+ `false`. — *end note*]
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+
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+ #### Member functions <a id="time.clock.gps.members">[[time.clock.gps.members]]</a>
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+
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+ ``` cpp
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+ static time_point now();
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+ ```
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+
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+ *Returns:* `from_utc(utc_clock::now())`, or a more accurate value of
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+ `gps_time`.
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+
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+ ``` cpp
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+ template<class Duration>
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+ static utc_time<common_type_t<Duration, seconds>>
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+ to_utc(const gps_time<Duration>& t) noexcept;
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+ ```
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+
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+ *Returns:*
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+
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+ ``` cpp
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+ gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} + 315964809s
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+ ```
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+
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+ [*Note 1*:
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+
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+ ``` cpp
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+ 315964809s == sys_days{1980y/January/Sunday[1]} - sys_days{1970y/January/1} + 9s
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+ ```
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+
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+ — *end note*]
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+
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+ ``` cpp
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+ template<class Duration>
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+ static gps_time<common_type_t<Duration, seconds>>
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+ from_utc(const utc_time<Duration>& t) noexcept;
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+ ```
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+
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+ *Returns:*
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+
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+ ``` cpp
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+ gps_time<common_type_t<Duration, seconds>>{t.time_since_epoch()} - 315964809s
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+ ```
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+
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+ [*Note 2*:
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+
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+ ``` cpp
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+ 315964809s == sys_days{1980y/January/Sunday[1]} - sys_days{1970y/January/1} + 9s
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+ ```
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+
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+ — *end note*]
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+
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+ #### Non-member functions <a id="time.clock.gps.nonmembers">[[time.clock.gps.nonmembers]]</a>
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+
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+ ``` cpp
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+ template<class charT, class traits, class Duration>
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+ basic_ostream<charT, traits>&
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+ operator<<(basic_ostream<charT, traits>& os, const gps_time<Duration>& t);
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+ ```
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+
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+ *Effects:* Equivalent to:
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+
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+ ``` cpp
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+ return os << format(STATICALLY-WIDEN<charT>("{:%F %T}"), t);
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+ ```
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ auto st = sys_days{2000y/January/1};
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+ auto gt = clock_cast<gps_clock>(st);
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+ cout << format("{0:%F %T %Z} == {1:%F %T %Z}\n", st, gt);
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+ ```
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+
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+ Produces this output:
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+
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+ ``` text
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+ 2000-01-01 00:00:00 UTC == 2000-01-01 00:00:13 GPS
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+ ```
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+
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+ — *end example*]
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+
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+ ``` cpp
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+ template<class charT, class traits, class Duration, class Alloc = allocator<charT>>
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+ basic_istream<charT, traits>&
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+ from_stream(basic_istream<charT, traits>& is, const charT* fmt,
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+ gps_time<Duration>& tp, basic_string<charT, traits, Alloc>* abbrev = nullptr,
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+ minutes* offset = nullptr);
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+ ```
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+
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+ *Effects:* Attempts to parse the input stream `is` into the `gps_time`
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+ `tp` using the format flags given in the NTCTS `fmt` as specified in
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+ [[time.parse]]. If the parse fails to decode a valid date,
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+ `is.setstate(ios_base::failbit)` is called and `tp` is not modified. If
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+ `%Z` is used and successfully parsed, that value will be assigned to
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+ `*abbrev` if `abbrev` is non-null. If `%z` (or a modified variant) is
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+ used and successfully parsed, that value will be assigned to `*offset`
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+ if `offset` is non-null. Additionally, the parsed offset will be
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+ subtracted from the successfully parsed timestamp prior to assigning
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+ that difference to `tp`.
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+
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+ *Returns:* `is`.
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+