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From: Steven Watanabe (steven_at_[hidden])
Date: 2007-03-05 20:54:49


Deane Yang <deane_yang <at>> writes:

> F(d, v) = [(d/d_0)^{1/3}/ (v/v_0)^{sqrt(2)} - (v/v_0)^(-5/4}] * F_0
> where d_0 = 1 meter, v_0 = 1 meter/second, and F_0 = 1 newton.
> Then if your library has implicit unit conversions, then formula will be
> properly calculated, no matter what units the input values d and v are in.
> But surely I'm telling you anything new? I'm pretty sure I learned
> tricks like this from physicists. So I still need an example where this
> approach does not work or where the extra divisions cause a serious problem.
> Deane

If d_0, v_0, and F_0 are units instead of
quantities, everything will work perfectly
and there will be no extra divisions at
runtime when the quantity is already in the
correct system.

I can think of two cases where temorarily bypassing
quantity can be useful. The first is when you already
have a function that operates on the raw value_type.
Then you can simply write an overload taking a quantity
which forwards to that function.

The second case is when the value_type is a complex
UDT. In this case it may be possible to write
the function in a much more efficent way by
using it directly. Of course, the usual caveats about
optimization apply.

In Christ,
Steven Watanabe

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