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

From: Martin Bonner (Martin.Bonner_at_[hidden])
Date: 2007-02-09 09:21:01

----Original Message----
From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]] On Behalf Of Andreas Harnack
Sent: 09 February 2007 14:11 To: boost_at_[hidden]
Subject: Re: [boost] formal review request : mcs::units

> Michael Walter wrote:
> 
>> How would you store the products? I don't really like the idea of
>> unintuitive limits on the composition of base units (e.g. you can
>> represent 2^31, but not 19^31)
> 
> 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.

How would you deal with fractional powers of units?  (The subject came
up in the pqs/quan review, and it was clear they were significant to
some people).

-- 
Martin Bonner
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PI SHURLOK LTD
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