Re: [boost] [date_time] i/o formatting

From: Jeff Garland (jeff_at_[hidden])
Date: 2006-02-26 11:36:46

On Sun, 26 Feb 2006 08:36:02 -0500, Caleb Epstein wrote
> On 2/25/06, Jeff Garland <jeff_at_[hidden]> wrote:
> >
> > Shouldn't we stick to the current set of format strings which is
> > consistent
> > with the current standards?
> >
> >       cout << time_format("%H:%M:%s") << my_ptime << endl;
> 
> I'm a huge fan of strftime, but being built around "struct tm" it 
> has no way of knowing about fractional seconds and so there are no %-
> escapes for formatting them.  Do you have any thoughts on how a 
> format might be expressed for ptime, which can clearly be used to 
> carry a much higher resolution timestamp?

Yep, sure do :-) Here's a link to round 1 of my TR2 proposal (n1900) :

http://www.crystalclearsoftware.com/libraries/date_time/proposal_v75/date_time_standard.html#design.extensions_to_time_put_and_format_flags

These are the same ones in boost date-time.  Specifically w.r.t. fractional
seconds date-time uses %s to include seconds and fractional seconds and %S to
be seconds only for all time_duration and ptime formatting.  Finer grained
control is allowed by using %f and %F which have different behaviors when the
fractional seconds are 'zero'. The only thing this system doesn't allow you to
control is the number of fractional digits to display which would need
something like %2f. Some of these flags conflict with various 'native'
versions of strftime, but most are extensions of the current standard.  

As for 'struct tm', it is clearly a problem.  My recommendation for a
replacement can be found here:

http://www.crystalclearsoftware.com/libraries/date_time/proposal_v75/date_time_standard.html#design.ymd_types

except that there is a bug in that doc because fractional second count need to
be a long at least and probably should be a unique type:

//struct to represent elements of a point in time
struct timepoint {
  year_type  year;        //32 bit unsigned integer -- range depends on calendar
  week_type  month;       //short integer 1-12 -- zero flags invalid
  short      day_of_year; //short 
  short      day_of_week;  //0-6  -- 0 == sunday
  short      week_number;  //1-53 - 0 indicates invalid
  short hours;             //0-24
  short minutes;           //0-59
  short seconds;           //0-60 -- 60 is leap second
  frac_seconds_type fractional_seconds_count; 
  short frac_seconds_resolution;  //increments of 10 only
};

Basically tm without the any of the text fields.  This structure should be a
able to drive all the needed formatting of dates and times.  Note that
different resolution time systems are supported.

Jeff