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3270. Parsing and formatting %j with durations

Section: 29.12 [time.format], 29.13 [time.parse] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25

Priority: 2

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Discussion:

%j represents the day number of the year when formatting and parsing time_points. It is also handy to interpret this flag consistently when formatting and parsing durations. That is if there is not enough information in the stream to represent a time_point, and if the target of the format/parse is a duration, %j represents a number of days. 

#include <chrono>
#include <iostream>
#include <sstream>
#include <string>

int main()
{
  using namespace std;
  using namespace std::chrono;
  // Parse %j as number of days into a duration
  istringstream in{"222"};
  hours d;
  in >> parse("%j", d);
  cout << d << '\n';
  cout << format("{:%j}", d) << '\n';
}

Output: 

5328h
222

[2019-10 Priority set to 2 after reflector discussion]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

  1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

    Table 98 — Meaning of conversion specifiers [tab:time.format.spec]
    Specifier Replacement
    […]
    %j The day of the year as a decimal number. Jan 1 is 001. If the result is less than three
    digits, it is left-padded with 0 to three digits. If the type being formatted is a
    specialization of duration, it is formatted as a decimal number of days.
    […]

  2. Modify "Table 99 — Meaning of parse flags" [tab:time.parse.spec] as indicated:

    Table 99 — Meaning of parse flags [tab:time.parse.spec]
    Flag Parsed value
    […]
    %j The day of the year as a decimal number. Jan 1 is 1. The modified command %Nj
    specifies the maximum number of characters to read. If N is not specified, the default
    is 3. Leading zeroes are permitted but not required. If the type being parsed is a
    specialization of duration, it is parsed as a decimal number of days.
    […]

[2020-02-13 After Thursday afternoon discussion in Prague, Marshall provides updated wording.]

[2020-02 Status to Immediate on Thursday night in Prague.]

Proposed resolution:

This wording is relative to N4830.

  1. Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:

    Table 98 — Meaning of conversion specifiers [tab:time.format.spec]
    Specifier Replacement
    […]
    %j If the type being formatted is a specialization of duration, the decimal number of days
    without padding. Otherwise, the    The day of the year as a decimal number.
    Jan 1 is 001. If the result is less than three digits, it is left-padded with 0 to three digits.
    […]

  2. Modify "Table 99 — Meaning of parse flags" [tab:time.parse.spec] as indicated:

    Table 99 — Meaning of parse flags [tab:time.parse.spec]
    Flag Parsed value
    […]
    %j If the type being parsed is a specialization of duration,
    a decimal number of days. Otherwise, the    The day of the year as a decimal number. Jan 1 is 1.
    In either case, theThe modified command %Nj specifies the maximum number of characters to read.
    If N is not specified, the default is 3. Leading zeroes are permitted but not required.
    […]