2008/5/27 David Abrahams <dave@boostpro.com>:
The one property of boost::date_time that knocked my socks off was the default constructor that 'is-not-a-date-time'. I paraphrase this as: boost::time is lacking a big bang ;-)
Default constructors in nearly all the types that I've seen in my lifetime (I know the realm of my unknown unknowns is vast ;-) are intended to be elements of the type they do construct.
Well, sure, but the built-ins in C++ generally default construct as singular "degenerate" elements that you can assign to and destruct, and nothing else.
My generic library code has no problem with the 'value nature' of built in types. Because from the initial default constructed value unsigned int x = int(); // x = 0 you can reach ++x // x = 1 ... ++x // x = 2^32 and thus all values of the types set of values. This is not the case for boost::date_time objects like e.g. ptime ptime x = ptime(); // x = "not-a-date-time" // ++x error ++ not available x += ptime::time_duration_type::unit(); // x = "not-a-date-time" ... x += ptime::time_duration_type::unit(); // x = "not-a-date-time" no element of the set of values of ptime is reachable from the default ctor with any available ptime operations or functions.
Moreover in so many cases including built in types, default ctors serve as *initial elements*. Other elements can be *reached* from T() applying operations of T on them.
This applies to all built in types
You must be thinking of value-initialization. Please read http://www.boost.org/doc/libs/1_35_0/libs/utility/value_init.htm
This is helpful in general. Yet, as far as I can see no remedy for the singularity default constructor problem that I had with boost::date_time value_initialized<ptime> x; // x = "not-a-date-time" // ++x error ++ not available x += ptime::time_duration_type::unit(); // x = "not-a-date-time" ... x += ptime::time_duration_type::unit(); // x = "not-a-date-time" same problems. I am not yet able to discover the advantages of such default ctor construction especially if it derails generic code that works well with standard types. Regards Joachim --- Interval Template Library (ITL) download from http://sourceforge.net/projects/itl documentation http://www.herold-faulhaber.de/