search

tomaszkam / date

A date and time library based on the C++11/14/17 <chrono> header
Other
0 stars 0 forks source link

[LWG3230] Format specifier %y/%Y are missing locale alternative versions #2

Closed tomaszkam closed 5 years ago

tomaszkam commented 5 years ago

Original message:

%y format needs to document %Ey and %Oy. %Y format needs to document %EY.

tomaszkam commented 5 years ago

@HowardHinnant As far I understand this could be potentially handled as an extension post C++20 - adding this specifier would not be breaking.

HowardHinnant commented 5 years ago

Actually we need these now because strftime has them, and we can copy that:

%EY is replaced by the locale’s full alternative year representation. %Ey is replaced by the offset from %EC (year only) in the locale’s alternative representation. %Oy is replaced by the last 2 digits of the year, using the locale’s alternative numeric symbols.

tomaszkam commented 5 years ago

Same as for the other issue, how we would present it as issue, not new design (extension)? I think in this case, we call constiency with strftime.

tomaszkam commented 5 years ago

Discussion: The year format specifier ('y', 'Y') are missing the locale alternative version ('%EY', '%Ey' and '%Oy'). That makes it inconsistent with the POSIX strftime specification (per http://pubs.opengroup.org/onlinepubs/009695399/functions/strftime.html):

%Ey Replaced by the offset from %EC (year only) in the locale's alternative representation. %EY Replaced by the full alternative year representation. %Oy Replaced by the year (offset from %C ) using the locale's alternative numeric symbols.

and parse specifiers (http://eel.is/c++draft/time.parse) that accepts these modified command.

Drafting note: For the '%Oy' specifier we preserve consistency with the current specification for '%Od' and '%Oe' from
"Meaning of format conversion specifier" (http://eel.is/c++draft/time.format#tab:time.format.spec):

The modified command %Od produces the locale's alternative representation. The modified command %Oe produces the locale's alternative representation.

, as their corresponding POSIX specification is matching one for %Oy:

%Od Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading zeros if there is any alternative symbol for zero; otherwise, with leading spaces. %Oe Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading spaces.

Proposed wording: Change the '%y' entry in the "Table 87 — Meaning of format flags" as follows: The last two decimal digits of the year. If the result is a single digit it is prefixed by 0. The modified command %Oy produces the locale's alternative representation. The modified command %Ey produces the locale’s alternative representation of offset from %EC (year only).

Change the '%Y' entry in the "Table 87 — Meaning of format flags" as follows: The year as a decimal number. If the result is less than four digits it is left-padded with 0 to four digits. The modified command %EY produces the locale's alternative full year representation.