Re: [boost] Re: new compile-time dimensional analysis and unitlibrary

From: Matthias Schabel (boost_at_[hidden])
Date: 2003-12-09 21:07:45

> I click on http://groups.yahoo.com/group/boost/files/YANL.zip
> and I get a yahoo groups page saying,
>
> "The requested file or directory is not found on the server."

Hmmm...it's working for me.  Anyone else having problems?

> I got sources I wrote years ago for angle types and trig functions.   
> My primary intent was speed, rather than portability, and most  
> functions are implemented in MMX/3DNow! inline assembly :), but they  
> work, and you might find useful stuff in them.  I defined also a type  
> of fixed.point arithmetic, mod 2pi angle type (that can accumulate  
> angle differentials ad-infinitum without losing precision) and that  
> re-computes and caches sine and cosine values, for use with angles  
> that seldom change, but often need trig values taken from.

As it stands, the library is capable of wrapping any well-behaved  
arithmetic type, so as long as those classes support the basic
operations necessary, it should be no problem to add the  
zero-runtime-overhead unit checking to them.  See in particular the  
implementation in measurement.hpp and its use in test_units.cpp for an  
example.

>  My question about angles was a bit philosophical, though: They use  
> units (cycles, quarter-cycles, radians, degrees) but they are  
> dimension-less.  Are there any other such animals, besides angles?

I guess there is sort of a dichotomy of definitions.  Formally, for  
physical measurables, the seven basic quantities are length, mass,  
time, quantity, temperature, current, and luminous intensity (or any  
linear combination thereof).  However, as you note, in practical  
situations you may want to keep track of other things - the library  
allows dimensional manipulations of arbitrary tag types for precisely  
this reason.  Angles are really a ratio of two lengths - the distance  
travelled along a circular perimeter relative to the total perimeter of  
the circle, as contrasted with, for example, solid angle which is the  
ratio of two areas.  In fact, if the universe was spherical, length and  
angle would be interchangeable.  Of course, you may also want to keep  
track of things like quantity_apples/quantity_oranges - important if  
you're a grocer even though they're technically not dimensionally  
distinct...

Matthias

------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
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mschabel at ucair med utah edu