Re: [boost] [Review][PQS] Review deadline

From: Leland Brown (lelandbrown_at_[hidden])
Date: 2006-06-07 17:39:25

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