Re: [boost] formal review request : mcs::units

From: james.jones_at_[hidden]
Date: 2007-02-09 11:29:27

From: Andreas Harnack <ah.boost.02_at_[hidden]>
> I might have been a bit imprecise here, so please consider:
> 
> 
> 	const unsigned int Time 	= 2;
> 	const unsigned int Length	= 3;
> 	const unsigned int Mass 	= 5;
> 	const unsigned int Current	= 7;
> 	const unsigned int Temp 	= 11;
> 	const unsigned int Amount	= 13;
> 	const unsigned int Intensity	= 17;
> 
> 	template <unsigned int N, unsigned int D=1>
> 		struct Rational { ... };
> 
> 	typedef Rational<Time> time;
> 	typedef Rational<Length> length;
> 	typedef Rational<Mass*Length,Times*Times> force;
> 	typedef Rational<Mass*Length*Length,Times*Times> energy;
> 
> 	quantity<energy, double> e;
> 
> so the representation of energy would be just 5*3*3/2*2 = 45/4.
> 
> You're right, there is a computational limit, but I wouldn't expect to
> a see a dimension with the power of 31. Exponends of 4 are about the 
> highest I've ever seen, and (2*3*5*7*11*13*17)^3 still fits in 57 bits, 
> so we might want to use long or even long long unsigned ints, but that 
> should be fine for most situations.

If exponents of 4 are the highest you've seen, shouldn't you consider the size of (2*3*5*7*11*13*17)^4, which requires 76 bits? Also, I may want to extend the list - some people have requested a "money" unit, for example, which would be assigned the value 19 in your system. Then I can't even represent all powers of 3. I suspect this idea, while clever, does not cover the problem domain sufficiently well to be superior to lists.

-
James Jones             Administrative Data Mgmt.
Webmaster               375 Raritan Center Pkwy, Suite A
Data Architect          Edison, NJ 08837