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Date: Thu, 09 Jan 2003 22:26:12 -0500
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Why is Ada NOT a good choice for a beginner to programming?
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Date: 2003-01-09T22:26:12-05:00
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Georg Bauhaus wrote:
> In which languages has this been fixed?

In C++. It's easy to define classes which are nothing more than
a wrapper around an underlying numeric type, are the same size
as that type, but which also carry around unit information, so
that the product of two lengths is an area. It looks something
like this:

template<typename T, int Mass, int Length, int Time>
struct Unit
{
	T v;
	Unit(T v) : v(v) { }
	// Addition and subtraction are defined only for identical units
	Unit operator+(Unit o) { return Unit(v + o.v); }
	Unit operator-(Unit o) { return Unit(v - o.v); }
};

// Multiplication and division are defined for any units

template<typename T, int M1, int L1, int T1, int M2, int L2, int T2>
inline Unit<T, M1 + M2, L1 + L2, T1 + T2>
operator*(Unit<T,M1,L1,T1> l, Unit<T,M2,L2,T2> r)
{ return Unit<T, M1 + M2, L1 + L2, T1 + T2>(l.v * t.v); }

template<typename T, int M1, int L1, int T1, int M2, int L2, int T2>
inline Unit<T, M1 - M2, L1 - L2, T1 - T2>
operator/(Unit<T,M1,L1,T1> l, Unit<T,M2,L2,T2> r)
{ return Unit<T, M1 - M2, L1 - L2, T1 - T2>(l.v / t.v); }

typedef Unit<double, 0, 1, 0> Length;
typedef Unit<double, 0, 2, 0> Area;
Area compute(Length l, Length r) { return l * r; }