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local_info struct

  • Artikkel

Provides a low-level interface to time zone information about the result of converting a local_time to a sys_time.

Syntax

struct local_info;

Members

Function Description
result A code describing the result of the conversion.
first, second The result of the conversion.

Member constants

Name Value Description
unique 0 The result of the conversion is unique.
nonexistent 1 There isn't a corresponding sys_time to convert the local_time to.
ambiguous 2 The result of the conversion is ambiguous.

Non-members

Function Description
operator<< Output a local_info to a stream.

result

The result of converting a local_time to a sys_time.

int result;

Return value

The result can be one of the following values:

  • unique: The local_time was successfully converted to a sys_time.
  • nonexistent: There isn't a mapping from the local_time to the sys_time.
  • ambiguous: The local_time lands during a transition between daylight saving time and standard time and so could be converted to two sys_time values.

Remarks

For information about how first and second are set based on result, see the following section about first, second.

first, second

Provides the result of converting a local_time to a sys_time.

sys_seconds begin;
sys_seconds end;

Return value

first and second are set based on result as follows:

result first second
unique Contains the value of local_time converted to sys_time zero-initialized
nonexistent A sys_info that ends just before the local_time A sys_info that begins just after the local_time.
ambiguous A sys_info that ends just after the local_time A sys_info that starts just before the local_time.

ambiguous result code

During the transition from daylight saving time to standard time in the fall, clocks essentially gain an extra hour. This can be confusing because doesn't the transition to standard time mean losing an hour? By falling back an hour, the hour before the transition will be repeated after the clock adjusts for standard time. Consider the change to standard time in New York, which happens on the first Sunday in November at 2:00am. First, 1:00am goes by. At 2am, the clock transitions to standard time, so now it's 1:00am again. That means the time between 1am and 2am will be "repeated", effectively adding an hour. See ambiguous_local_time for an example.

If a local_time specifies a time during this "extra" hour, it isn't clear how to convert it. Should the converted time be treated as the "first" time that hour happens, or the "second"? If the enum choose isn't specified to indicate which it should be, expect result to be set to ambiguous, and first and second will reflect the two choices for what the time the local_time could be converted to.

nonexistent result code

During the transition from standard time to daylight saving time in the spring, clocks essentially lose an hour. This can be confusing because doesn't the transition to daylight saving time mean adding an hour? By "springing forward" an hour, the hour following the transition is effectively removed. Consider the change to daylight saving time in New York, which happens on the second Sunday in March at 2am. At 2am, the clock transitions to daylight savings time and now reads 3:00am. If the local_time being converted is 2:30am, for example, that time is during the period that was "removed" and so you can expect result to have the nonexistent code. See nonexistent_local_time for an example.

Example: get a local_info

// compile using: /std:c++latest
#include <iostream>
#include <chrono>

using namespace std::chrono;

int main()
{
    const auto& timeZoneDatabase = get_tzdb();
    const auto& currentZone = timeZoneDatabase.current_zone();

    local_time<system_clock::duration> lt = currentZone->to_local(system_clock::now());

    auto localInfo = currentZone->get_info(lt);

    std::cout << "local_time: " << lt << "\n";
    std::cout << localInfo << "\n";

    return 0;
}

local_time: 2021-09-08 15:37:57.6853963
result: unique, first: (begin: 2021-03-14 10:00:00, end: 2021-11-07 09:00:00, offset: -25200s, save: 60min, abbrev: PDT)

Requirements

Header: <chrono>

Namespace: std::chrono

Compiler Option: /std:c++latest

<chrono>
ambiguous_local_time exception
choose enum
file_clock class
high_resolution_clock
local_t struct
non_existent exception
system_clock struct
tai_clock class
utc_clock class
Header Files Reference