29 Time library [time]

29.11 Time zones [time.zone]

29.11.4 Information classes [time.zone.info]

29.11.4.1 Class sys_info [time.zone.info.sys]

namespace std::chrono { struct sys_info { sys_seconds begin; sys_seconds end; seconds offset; minutes save; string abbrev; }; }
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A sys_info object can be obtained from the combination of a time_zone and either a sys_time or local_time.
It can also be obtained from a zoned_time, which is effectively a pair of a time_zone and sys_time.
2
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[Note 1: 
This type provides a low-level interface to time zone information.
Typical conversions from sys_time to local_time will use this class implicitly, not explicitly.
— end note]
3
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The begin and end data members indicate that, for the associated time_zone and time_point, the offset and abbrev are in effect in the range [begin, end).
This information can be used to efficiently iterate the transitions of a time_zone.
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The offset data member indicates the UTC offset in effect for the associated time_zone and time_point.
The relationship between local_time and sys_time is: offset = local_time - sys_time
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The save data member is extra information not normally needed for conversion between local_time and sys_time.
If save != 0min, this sys_info is said to be on “daylight saving” time, and offset - save suggests what offset this time_zone might use if it were off daylight saving time.
However, this information should not be taken as authoritative.
The only sure way to get such information is to query the time_zone with a time_point that returns a sys_info where save == 0min.
There is no guarantee what time_point might return such a sys_info except that it is guaranteed not to be in the range [begin, end) (if save != 0min for this sys_info).
6
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The abbrev data member indicates the current abbreviation used for the associated time_zone and time_point.
Abbreviations are not unique among the time_zones, and so one cannot reliably map abbreviations back to a time_zone and UTC offset.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const sys_info& r);
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Effects: Streams out the sys_info object r in an unspecified format.
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Returns: os.

29.11.4.2 Class local_info [time.zone.info.local]

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namespace std::chrono { struct local_info { static constexpr int unique = 0; static constexpr int nonexistent = 1; static constexpr int ambiguous = 2; int result; sys_info first; sys_info second; }; }
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[Note 1: 
This type provides a low-level interface to time zone information.
Typical conversions from local_time to sys_time will use this class implicitly, not explicitly.
— end note]
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Describes the result of converting a local_time to a sys_time as follows:
  • (2.1)
    When a local_time to sys_time conversion is unique, result == unique, first will be filled out with the correct sys_info, and second will be zero-initialized.
  • (2.2)
    If the conversion stems from a nonexistent local_time then result == nonexistent, first will be filled out with the sys_info that ends just prior to the local_time, and second will be filled out with the sys_info that begins just after the local_time.
  • (2.3)
    If the conversion stems from an ambiguous local_time, then result == ambiguous, first will be filled out with the sys_info that ends just after the local_time, and second will be filled out with the sys_info that starts just before the local_time.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const local_info& r);
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Effects: Streams out the local_info object r in an unspecified format.
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Returns: os.