[Boost-users] [units] conversion factor for non-base

Subject: [Boost-users] [units] conversion factor for non-base
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2010-04-20 15:31:44

I'm unsure how to go about what I need.  I've looked at the library code 
and the documentation and am not even sure what I want is possible, 
though it should be.

In most of our code we use the si system and have defined composite 
dimensions for most of what is unique to our application.  However, 
there is one dimension where there is no standard unit in the si system: 
flow coefficient.

The flow coefficient is Q/sqrt(dP), a volumetric flow/sqrt(pressure).  
We have derived and defined a unit that would be the si standard for 
this quantity if there was one and things work fine.  I have a runtime 
unit system that then converts between user values that make sense, to 
our internal si version that does not but works.

However, now I want to write some alternative functions that use the 
actual units that engineers really use: Cv and Kv.  I would like these 
alternative functions to then convert into this si unit we've created 
but the interface should distinguish between Cv, Kv, and my made up 
unit.

The first part is easy: make a system that only contains this funky 
dimension and then use it to make quantities that are not in the si 
system.

The second part is what I can't find.  The conversion templates in 
conversion.hpp seem to specifically expect base units which at least the 
si part is not and my conversions should use my funky si unit as their 
basis.  So the question I have is how can I write conversions such that 
I can do this:

quantity<si::flow_coefficient> fc(42 * Kv);

and this:

void f(quantity<Kv::coefficient>);
void f(quantity<Cv::coefficient>);
void f(quantity<si::coefficient>);

Where Kv and Cv are base units in their own systems.

The problem is sort of like degrees and radians, which I was going to 
use as a basis when I first was looking at this issue, except that in 
that case the unit is a base unit for a base dimension and in my case 
I'm working with a derived dimension within the si system.

The only way I can think of to do it is to make a system with the 
components within the derived dimension and make conversion factors for 
those.  The problem is that I am not sure such systems can be coherent 
and it would be a significant effort above and beyond the direct 
conversion factors I already know.

-- 
http://crazyeddiecpp.blogspot.com/