[boost] Re: Physical Quantities revisited

From: Jan Langer (jan_at_[hidden])
Date: 2003-10-24 18:17:03

Hugo Duncan wrote:
> if we have
> 
>   quantity<NumericType, Unit, Dimension>
> 
> then we can have a trait to determine whether or not the Unit has any
> sort of trnsform asociated with it.  The Unit could then provide the
> (possibly static) functions required to perform the transform.

in general i think we should stick to the idea that everything is stored 
and calculated in the fundamental unit. it should be made easy to change 
the fundamental unit. then everyone who needs a different internal 
scale, must say beforehand that he wants to use such a system. something 
like:
        
typedef units_system <si, light_years> my_system;

quantity <double, my_system::length> l1 (12); // 12 light_years

// something quite small
quantity <double, my_system::length> l2 = 1 * meter;

------------------------------------

if the other path is followed, i'm also strongly in favor of hugo's 
idea, because it allows to do more complicated conversions between 
units. it could for example solve the celsius/fahrenheit issue.

but a problem might be to avoid unneccessary conversions. for example 
between two non-fundamental conversions like km to inches. both units 
know their conversion to meters. but i dont know if it might be an issue 
if it multpiles with 1000 and then divides by 0.0254.

and what should be the return type of an addition of kilometers and inches.
jan

-- 
jan langer ... jan_at_[hidden]
"pi ist genau drei"