[boost] Re: runtime unit-modifiable physical-quantitiesusingpqs1-00-01

From: Dan W. (danw_at_[hidden])
Date: 2004-01-03 19:34:02

> But but but....I think the point I am trying to make (unsuccessfully) is
> that a properly designed units library should not actually define any 
> units itself. The units should be created only by the user of the 
> library. Then there should be libraries derived from the units library,
> like the physical dimensions library.
> 
> I see nothing wrong with focusing on a particular use case when 
> designing the library, but if you want a truly useful C++ library,
> you want to design its guts so that they can be used by the library
> user to define whatever units the user happens to need. I think it
> is an error for the library writer to try to predict what units
> the library user needs.

I second that, 100%. Physical dimensions are not a magical set, even if 
they happen to map very well with known physical laws;  so what?  One 
might want to invent a Virtual UN-Reality Engine that has a completely 
different system of physical laws;  where 'ectoplasmic density' is a 
daily weather concern.  Without even going that far, how does one 
express UV-ratings, wind-chill factor, radiation or aminoacid balance in 
SI units?

I'm sure someone might be able to workout an SI-units equivalent for 
Roentgents, but if I use them in a program, I probably don't even care 
how many ergs root wavelength per square cm per second + particle 
momentum factor that is, since I'd probably be using Roengens in 
isolation just to keep logs for the EPA or whatever, in which case all I 
want is a float, not even a double, that merely doesn't accidentally 
convert itself to a wind-chill factor.