Re: [boost] [units] Crazy idea/question

From: Deane Yang (deane_yang_at_[hidden])
Date: 2007-02-16 21:51:35

Matthias Schabel wrote:

>> 1) I do not need a library that insists on defining some "system of
>> units" to which everything gets converted.
> 
> A major design goal of mcs::units was to allow users to work in a system
> that matches their problem. These can be physical units, but can be
> anything else, also...
> 

But I don't want the user of the library to have to define a "system" of 
units just to use a function in the library. If there is, say, a 
function that takes a distance, a mass, and a time and does computations 
with them, the user should be able to feed into that function arbitrary 
units for each of the three without having to use the dimensions library 
to define a system first.

So the thing I didn't like in your version of my sample code is the need 
for a template parameter called "System".

I think in another post you yourself indicate how these 
defined-by-convention "systems" of units used by physicists cause some 
awkwardness, because a system might not define the units for momentum to 
be equal to the units of mass times the units of distance divided by the 
units of time, creating the need for extra conversions.

I am pretty badly prejudiced as a mathematician. I think within a single 
code module, one should forget these arbitrary man-made conventions and 
code using mathematically consistent units (so the units for the product 
of two dimensions are always the product of the the units of the two 
dimensions, etc.). This yields in my opinion much cleaner mathematical 
formulas with simpler constants and therefore cleaner code as well. My 
view is that conventions like SI units should appear only in the 
interface to the module, and the first thing that the module should do 
on entry is to convert everything to a mathematically consistent set of 
units, rather than a set conforming to a rather arbitrary convention.

(I'm sorry if I sound like a closed-minded cranky old mathematician, but 
that's exactly what I am.)