Gerhard Wesp <gwesp <at> google.com> writes:
I think that handling of dimensional quantities and conversion factors are orthogonal concepts and should be separated.
An interesting idea. It might also help with some of the confusion about the purpose of the library.
I suggest that for maximum transparency the library should *exclusively* handle quantities expressed in SI units.
Conversion factors between non-SI units and SI units should be constant dimensional quantities, e.g. (assuming constructors from double):
const length foot = .3048 ; // Meter const power european_horse_power = 735.4987 ; // Watt const mass pound = 0.4535924; // Kilogram // ...
This way, one could e.g. construct dimensional quantities like this: const power deux_chevaux = 2 * european_horse_power;
Yes! That is the way I like to see construction of dimensional quantities. Cf my example:
length x = 1000.0 * meter; velocty v = 2.0 * nautical_mile / hour;
BUT - the difference is, I would like to see the compiler enforce this sort of strong typing and self-documenting for SI units also (as in my "meter" example above). Otherwise, if SI quantities can be constructed directly from double, nothing stops me from doing: force f = 10.0; // kg which is an undetected error because I apparently intended kg which is not a force unit. I prefer requiring: force f = 10.0 * kilogram; // error will be caught by compiler! and const length foot = .3048 * meter; FWIW, when I wrote the equivalent of the (so-called for now) t2_quantity, I implemented it exactly as you describe, keeping all quantities exclusively in SI units, but kept that fact hidden from the user in order to prevent direct conversions from built-in types and require the user to document his units, SI or otherwise. Likewise, similarly to your other example:
length altitude; double altitude_ft = altitude / foot;
I would write: double altitude_m = altitude / meter; if I needed the value as a nondimensional "double." -- Leland