Re: [boost] [Review] Quantitative Units library review begins today March 26

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-03-28 13:38:47

> How do you implement conversions between units that require more
> complicated arithmetic expressions (e.g. units of temperature)?
> Conversions for the vast majority of units are simply multiples of  
> each
> other and the conversion can be expressed at compile-time and
> multiplied by a runtime quantity value:

Take a look at unit_example_20 to see a demonstration of implementing  
temperature conversions (both absolute and relative). It turns out  
that relative temperature conversions are trivial (converting a  
temperature difference of 32 degrees Fahrenheit to Kelvin just  
involves the scale factor of 5/9). Absolute temperatures are the  
issue. Basically, at this point the library assumes that all  
quantities are relative as that makes the default behavior the most  
sensible. While it is probably worthwhile to have the absolute<>  
concept (also could be point<>) as a part of boost::numeric, I don't  
think it is really something that properly belongs in the unit  
library itself; hence it is only an example... You can achieve  
basically arbitrary conversions between quantities by specializing  
the conversion_helper<quantity1,quantity2> class...

Matthias