Joachim Faulhaber wrote:
only syntactically an Addability, semantically rather Concatenatablity (strange word ;-) But you are right. I focused too much on integral numerical types.
Trying to figure out what, qualitatively, makes a binary operation "addition" is a hard question, and probably out scope here, but I digress. :)
The date and time implementations that I've seen so far are all based on integral numeric types including boost::date_time. Implicit in this implementation is the property of a least incrementable unit. Based on this property incrementation and decrementation can always be implemented. To me this seems to be a rather universal property and not only detail of the implementing private member.
Actually, Visual Basic 6 uses a double to store DateTimes, with the integral portion representing date and the fractional portion representing time. Of course, Visual Basic 6 isn't what I'd recommend to use as a model for language design. I think it would make more sense to use the date/time iterators in Boost.DateTime (which I just now learned of!). They allow incrementing/decrementing with respect to a particular unit of measure, which lets the user completely ignore the underlying implementation of the date_time object. - Jim