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3270. Parsing and formatting%j withdurations

Section: 30.12[time.format], 30.13[time.parse]Status:C++20Submitter: Howard HinnantOpened: 2019-09-02Last modified: 2021-02-25

Priority:2

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Discussion:

%j represents the day number of the year when formatting and parsingtime_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 atime_point, and if the target of the format/parse is aduration,%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:

5328h222

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

Previous resolution [SUPERSEDED]:

This wording is relative toN4830.

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 is001. If the result is less than three digits, it is left-padded with0 to three digits.If the type being formatted is a specialization ofduration, it is formatted as a decimal number ofdays.
   […]

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

   Table 99 — Meaning ofparse flags [tab:time.parse.spec]

   Flag	Parsed value
   […]
   %j	The day of the year as a decimal number. Jan 1 is1. The modified command%Nj specifies the maximum number of characters to read. IfN is not specified, the default is3. Leading zeroes are permitted but not required.If the type being parsed is a specialization ofduration, it is parsed as a decimal number ofdays.
   […]

[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 toN4830.

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 ofduration, the decimal number ofdays without padding. Otherwise, theThe day of the year as a decimal number. Jan 1 is001. If the result is less than three digits, it is left-padded with0 to three digits.
   […]

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

   Table 99 — Meaning ofparse flags [tab:time.parse.spec]

   Flag	Parsed value
   […]
   %j	If the type being parsed is a specialization ofduration, a decimal number ofdays. Otherwise, theThe day of the year as a decimal number. Jan 1 is1. In either case, theThe modified command%Nj specifies the maximum number of characters to read. IfN is not specified, the default is3. Leading zeroes are permitted but not required.
   […]