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

From: Andreas Harnack (ah.boost.02_at_[hidden])
Date: 2007-02-09 09:10:57

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.

Andreas