If you're going to use user-defined literals for things like "mm" and "km", then you really want to combine it with something like Boost.Units. Such a library doesn't yet exist. (See
http://stackoverflow.com/questions/9257826/physical-boost-un... for an example of roll-your-own.)
Today I wanted to convert from hPa to Torr. Pressure units are diverse. At least I didn't need psig.