* Ideally, you want separate types for absolute temperatures and temperature-deltas. Absolutes cannot be added, multiplied, or divided; subtracting absolutes produces a delta; adding or subtracting an absolute from a delta produces an absolute; deltas can be added and subtracted, and multiplied by unitless values.
* Multiplying a MeterNum by a MeterNum would ideally produce a SquareMeterNum.
* If you don't have to worry about precision, MeterNum and FootNum should both be replaced by LengthNum.
* You can convert a LengthNum into, say, meters by dividing by whatever value represents "1 meter" to produce a unitless int.
* Often operators don't make sense at all. (a << b) only makes sense if b is a unitless integer, for example.
I implemented most of this (but not absolute temperatures) as a Python library. The source is at <http://timmaxwell.org/svn/src/pychem/units.py>; and <http://timmaxwell.org/svn/src/pychem/std_units.py>. (Warning: these will download rather than opening in the browser.) I also found this: <http://pypi.python.org/pypi/units/>.