Re: [Boost-users] [Units] Un-scaling dimensionless value

Subject: Re: [Boost-users] [Units] Un-scaling dimensionless value
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-02-18 00:22:45

On 18/02/2014 11:18, Quoth Lindley French:
> I also think it's a design flaw to allow dimensionless units to have a
> scale factor, by the way. Ideally, that would be backed out
> automatically in the dimensionless case.

I must admit that one can be quite annoying.  There's a default 
conversion between an unscaled dimensionless quantity and the value type 
of the quantity but this is not valid for scaled dimensionless 
quantities.  There was a thread about this a little while ago.

It's not really a problem if you're doing all calculations using 
Boost.Units quantities and then simply printing the result, but it can 
bite you if you want to pass that as a raw value to some external API 
(which is probably the more common case in real-world code).

I think the "right way" to do it (according to Boost.Units logic) is to 
define a system that has your target unit as a base unit, such that when 
you move the unit from one system to another it automatically rescales 
it to become the "real" value that you actually want, and you can get an 
unscaled dimensionless value out of it.  It's a little annoying, but 
it's not actually all that much code.

(Though as Steven said this won't help in your case because rescaling 
requires doubles, and rational<> doesn't support that.)