Re: [boost] Re: Quantity libraries: SIunits

From: George A. Heintzelman (georgeh_at_[hidden])
Date: 2001-09-14 10:22:12

> I too am curious how this would come up.  I can think of function that
> would have a signature of:

> 	mass/volumn density(mass m, distance r)
> 
> but why would you want two fuctions:
> 
> 	kg/m^3  density(kg m, m r)
> 	kg/cm^3 density(kg m, cm r)

Sure, I can come up with an example. Suppose you want to know how much 
energy is being delivered by a particle accelerator. You're a doctor 
calculating a dose for a patient, so you want the energy either in ergs 
or calories (I'm not really sure what doctors will use, but it's not 
electron volts). On the other hand, the energy of the beam is typically 
measured with two quantities: first, the energy of a given particle, in 
some electron-volt scale, and the luminosity of the beam, in units 
which can be converted to particles per second (luminosity is more 
complicated than this, but let me finesse this for purposes of the 
example). Your calculation will be something like:

        energy/particle * particles/second * seconds of exposure

but you will want to implicitly convert from MeV/second to ergs/second.

I'm sure this is not the only example. There must be many examples 
where calculations need to cross domains in which the conventional 
units are different.

Now, SIunits handles this, as best as can tell, transparently and 
easily. That is IMHO how it should be. A programmer should not need to 
worry about whether her internal representation is in inches or 
centimeters or light-seconds, unless she is running up against 
representable range issues, and then she will work hard to get it 
right. SIunits allows you to do this with limited manual intervention 
needed,  by defining your own view, defining the conversion constants 
once from the fundamental representation. This is IMHO how it should be.

> So I see the need for having IO from and into various units.  But once
> inside C++, I'm not sure why you would mix units.

You need to have the ability to represent different units for 
representable range issues, primarily. Representing cosmological 
distances in femtometers or particle physics lengths in megaparsecs is 
going to run into trouble, especially when you cube lengths to get 
volumes.

Once you have the ability to represent measurements in different views, 
you naturally are going to have potential incompatibilities between 
different libraries which make different natural choices of those 
views, so supplying the conversions becomes important.

George