[boost] Re: runtime unit-modifiablephysical-quantitiesusingpqs1-00-01

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2004-01-05 07:31:43

Alan Bellingham wrote:
> Daniel Frey:
> 
> 
>>Wait! I think the above mixes up some important concepts. Look at this:
>>
>>length     <-> meter
>>time       <-> hour
>>speed      <-> mph
>>frequency  <-> hertz
>>
>>I hope you see what I mean. 'hertz' is IMHO totally different than 
>>length, time or speed.
> 
> 
> Only in that frequency is the inverse of period - and is therefore a
> 1/time unit.
> 
>  length     <-> meter
>  time       <-> second
>  speed      <-> meters/second
>  frequency  <-> /second
> 
> When you're doing dimensional analysis, you may multiply a length by a
> frequency to get a speed - for example, the wavelength of a particular
> radio band times its frequency should give you the speed of light.

Of course, but that's not a contradiction of what I said above. It just 
shows that there is more to a unit-library like relationship. To make 
this clear:

speed = length/time

(which you can't use that way directly, as C++-types don't work that 
way) and

mph = mile/hour

The former can probably be written like this:

typedef divide< length, time >::type speed;

But I have to admit that I haven't put enough thinking into units so 
far. I'd like to finish the constant libraries first (I'm working on the 
documentation) before having a closer look at units. As I said already, 
I think that meter, second and hertz are constants, so I think that 
there is also a connection between the two fields...

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial solutions & technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de