std::chrono::from_stream(std::chrono::duration)(3)

NAME
std::chrono::from_stream(std::chrono::duration) - std::chrono::from_stream(std::chrono::duration)

Synopsis
   Defined in header <chrono>
   template< class CharT, class Traits, class Rep, class Period,
   class Alloc = std::allocator<CharT> >
   std::basic_istream<CharT, Traits>&
   from_stream( std::basic_istream<CharT, Traits>& is, const CharT* fmt, (since C++20)
   std::chrono::duration<Rep, Period>& d,
   std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
   std::chrono::minutes* offset = nullptr );
   Attempts to parse the input stream is into the std::chrono::duration d, interpreted
   as the time of day since midnight, according to the format string fmt.
   Behaves as an UnformattedInputFunction, except that it has an unspecified effect on
   is.gcount(). After constructing and checking the sentry object, attempts to parse
   the input stream is into d according to the format string fmt. If the parse fails to
   decode a valid duration, is.setstate(std::ios_base::failbit) is called and d is not
   modified.
   If the format specifier %Z is used and successfully parsed, the parsed value is
   assigned to *abbrev if abbrev is not null. If the format specifier %z or a modified
   variant is used and successfully parsed, the parsed value is assigned to *offset if
   offset is not null.

Parameters
   is - an input stream
   fmt - a format string (see below)
   d - std::chrono::duration object to hold the parse result
   abbrev - if not null, pointer to an object that will hold the time zone abbreviation
            or name corresponding to the %Z specifier
   offset - if not null, pointer to an object that will hold the offset from UTC
            corresponding to the %z specifier
  Format string
   The format string consists of zero or more conversion specifiers and ordinary
   characters. Each ordinary character, excluding whitespace characters and the
   terminating null character, matches one identical character from the input stream,
   or causes the function to fail if the next character on the stream does not compare
   equal.
   Each whitespace character matches zero or more whitespace characters in the input
   stream.
   Each unmodified conversion specifier begins with a % character followed by a
   character that determines the behavior of the specifier. Some conversion specifiers
   have a modified form in which an E or O modifier character is inserted after the %
   character. Some conversion specifiers have a modified form in which a width
   parameter given as a positive decimal integer (shown as N below) is inserted after
   the % character. Each conversion specifier causes the matched characters to be
   interpreted as parts of date and time types according to the table below.
   A character sequence in the format string that begins with a % but does not match
   one of the conversion specifiers below is interpreted as ordinary characters.
   If from_stream fails to parse everything specified by the format string, or if
   insufficient information is parsed to specify a complete result, or if parsing
   discloses contradictory information, is.setstate(std::ios_base::failbit) is called.
   The following conversion specifiers are available:
   Conversion Explanation
   specifier
       %% Matches a literal % character.
       %n Matches one whitespace character.
       %t Matches zero or one whitespace character.
                        Year
              Parses the century as a decimal number. The width N specifies the
       %C maximum number of characters to read. The default width is 2. Leading
      %NC zeroes are permitted but not required.
      %EC
              The modified command %EC interprets the locale's alternative
              representation of the century.
              Parses the last two decimal digits of the year. If the century is not
              otherwise specified (e.g. with %C), values in the range [69, 99] are
       %y presumed to refer to the years 1969 to 1999, and values in the range
      %Ny [00, 68] are presumed to refer to the years 2000 to 2068. The width N
      %Ey specifies the maximum number of characters to read. The default width is
      %Oy 2. Leading zeroes are permitted but not required.
              The modified commands %Ey and %Oy interpret the locale's alternative
              representation.
              Parses the year as a decimal number. The width N specifies the maximum
       %Y number of characters to read. The default width is 4. Leading zeroes are
      %NY permitted but not required.
      %EY
              The modified command %EY interprets the locale's alternative
              representation.
                        Month
       %b
       %B Parses the locale's full or abbreviated case-insensitive month name.
       %h
              Parses the month as a decimal number (January is 1). The width N
       %m specifies the maximum number of characters to read. The default width is
      %Nm 2. Leading zeroes are permitted but not required.
      %Om
              The modified command %Om interprets the locale's alternative
              representation.
                        Day
       %d Parses the day of month as a decimal number. The width N specifies the
      %Nd maximum number of characters to read. The default width is 2. Leading
      %Od zeroes are permitted but not required.
       %e
      %Ne The modified commands %Od and %Oe interpret the locale's alternative
      %Oe representation.
                        Day of the week
       %a Parses the locale's full or abbreviated case-insensitive weekday name.
       %A
       %u Parses the ISO weekday as a decimal number (1-7), where Monday is 1. The
      %Nu width N specifies the maximum number of characters to read. The default
              width is 1. Leading zeroes are permitted but not required.
              Parses the weekday as a decimal number (0-6), where Sunday is 0. The
       %w width N specifies the maximum number of characters to read. The default
      %Nw width is 1. Leading zeroes are permitted but not required.
      %Ow
              The modified command %Ow interprets the locale's alternative
              representation.
                        ISO 8601 week-based year
   In ISO 8601 weeks begin with Monday and the first week of the year must satisfy the
   following requirements:
     * Includes January 4
     * Includes first Thursday of the year
       %g Parses the last two decimal digits of the ISO 8601 week-based year. The
      %Ng width N specifies the maximum number of characters to read. The default
              width is 2. Leading zeroes are permitted but not required.
       %G Parses the ISO 8601 week-based year as a decimal number. The width N
      %NG specifies the maximum number of characters to read. The default width is
              4. Leading zeroes are permitted but not required.
       %V Parses the ISO 8601 week of the year as a decimal number. The width N
      %NV specifies the maximum number of characters to read. The default width is
              2. Leading zeroes are permitted but not required.
                        Week/day of the year
       %j Parses the day of the year as a decimal number (January 1 is 1). The
      %Nj width N specifies the maximum number of characters to read. The default
              width is 3. Leading zeroes are permitted but not required.
              Parses the week number of the year as a decimal number. The first Sunday
              of the year is the first day of week 01. Days of the same year prior to
       %U that are in week 00. The width N specifies the maximum number of
      %NU characters to read. The default width is 2. Leading zeroes are permitted
      %OU but not required.
              The modified command %OU interprets the locale's alternative
              representation.
              Parses the week number of the year as a decimal number. The first Monday
              of the year is the first day of week 01. Days of the same year prior to
       %W that are in week 00. The width N specifies the maximum number of
      %NW characters to read. The default width is 2. Leading zeroes are permitted
      %OW but not required.
              The modified command %OW interprets the locale's alternative
              representation.
                        Date
       %D Equivalent to "%m/%d/%y".
       %F Equivalent to "%Y-%m-%d". If the width is specified, it is only applied
      %NF to the %Y.
              Parses the locale's date representation.
       %x
      %Ex The modified command %Ex interprets the locale's alternate date
              representation.
                        Time of day
              Parses the hour (24-hour clock) as a decimal number. The width N
       %H specifies the maximum number of characters to read. The default width is
      %NH 2. Leading zeroes are permitted but not required.
      %OH
              The modified command %OH interprets the locale's alternative
              representation.
              Parses the hour (12-hour clock) as a decimal number. The width N
       %I specifies the maximum number of characters to read. The default width is
      %NI 2. Leading zeroes are permitted but not required.
      %OI
              The modified command %OI interprets the locale's alternative
              representation.
              Parses the minute as a decimal number. The width N specifies the maximum
       %M number of characters to read. The default width is 2. Leading zeroes are
      %NM permitted but not required.
      %OM
              The modified command %OM interprets the locale's alternative
              representation.
              Parses the second as a decimal number. The width N specifies the maximum
       %S number of characters to read. The default width is 2. Leading zeroes are
      %NS permitted but not required.
      %OS
              The modified command %OS interprets the locale's alternative
              representation.
       %p Parses the locale's equivalent of the AM/PM designations associated with
              a 12-hour clock.
       %R Equivalent to "%H:%M".
       %T Equivalent to "%H:%M:%S".
       %r Parses the locale's 12-hour clock time.
              Parses the locale's time representation.
       %X
      %EX The modified command %EX interprets the locale's alternate time
              representation.

Miscellaneous
              Parses the locale's date and time representation.
       %c
      %Ec The modified command %Ec interprets the locale's alternative date and
              time representation.
              Parses the offset from UTC in the format [+|-]hh[mm]. For example -0430
              refers to 4 hours 30 minutes behind UTC and 04 refers to 4 hours ahead
       %z of UTC.
      %Ez
      %Oz The modified commands %Ez and %Oz parses the format [+|-]h[h][:mm]
              (i.e., requiring a : between the hours and minutes and making the
              leading zero for hour optional).
              Parses the time zone abbreviation or name, taken as the longest sequence
       %Z of characters that only contains the characters A through Z, a through
              z, 0 through 9, -, +, _, and /.

Return value
   is.

Example
// Run this code
 #include <chrono>
 #include <iostream>
 #include <locale>
 #include <sstream>
 int main() {
     std::istringstream is{"16:14:34"};
     is.imbue(std::locale("en_US.utf-8"));
     std::chrono::hours hh;
     is >> std::chrono::from_stream("%H:%M:%S", hh);
     is.fail()
         ? std::cout << "Parse failed!\n"
         : std::cout << hh.count() << '\n';
 }

Possible output:
 16

See also
   parse parses a chrono object from a stream
   (C++20) (function template)