Hi Colin, 2008/5/29, Colin Caughie <c.caughie@indigovision.com>:
To answer your question with a question: What actual date/time do you think a default-constructed date_time should be, and by what time interval should ++ increment it?
To me it seems that any answers to these two questions must be completely arbitrary, in which case I'd question the usefulness of defining them.
From my view as an author of generic code (in this case interval-template code) these questions have been quite determined:
Coding for instance equality of intervals with open and closed bounds, if a,b,c and d are values of a discrete type, like an integer based time, the assumption, that there is a least unit (resolution) on the values of that type is inevitable. (a, b] == [c, d] is true, if(++a==c && b == d) If the resolution is nanosec and I would increment in seconds the result would be wrong. In addition, for an object like an empty interval [1,0] you need to generically express nullness and oneness. Time is theoretically continuous but technically it's defined by a finite unit "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" (http://en.wikipedia.org/wiki/Second). the ++ here is not the second but the time of the transition between levels. Many date and time classes like boost::date_time classes are implemented via an unsigned integral. So there is a finest resolution on them and thus a smallest representable duration. Incrementation (operator ++) has to be the incrementaton of such smallest unit. Integer implemented date and time classes also have a minimal representable time. This is naturally given by the 0 value of the implementing unsigned integer. From the (limited) point of view of such a special implementation, this is the beginning of all time and thus the awesome event of a big bang ;-) This would be my favourite candidate for a default constructor. Of course the time origin is still a little arbitrary. Yet for my generic code any value between min_time and max_time-1 was workable, even a moving one (like today, which I won't prefer), but unfortunately not a singularity.
While I understand your initial distaste for the idea, a singular value actually seems like a pretty good way out.
I haven't looked at your interval library (although I probably will, it sounds interesting), but one idea for using it with date_time might be to wrap date_time objects in a class of your own, which defines its own "big bang" time and default increment interval, e.g. 1 second.
That's what I had to do to make the librares play together: itl/ptime_adapter.hpp: ------------------------------------------------------------- boost::posix_time::ptime operator ++(boost::posix_time::ptime& x) { return x += boost::posix_time::ptime::time_duration_type::unit(); } boost::posix_time::ptime operator --(boost::posix_time::ptime& x) { return x -= boost::posix_time::ptime::time_duration_type::unit(); } template<> inline boost::posix_time::ptime type<boost::posix_time::ptime>::neutron() { return boost::posix_time::ptime(boost::posix_time::min_date_time); } ------------------------------------------------------------- cheers Joachim Interval Template Library download from http://sourceforge.net/projects/itl documentation http://www.herold-faulhaber.de/