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From: Phil Richards (news_at_[hidden])
Date: 20040109 06:54:36
On Thu, 08 Jan 2004 13:44:19 0700, Matthias Schabel wrote:
>> I'm not sure I understand the need for an absolute reference. Couldn't
>> each unit BE its own absolute reference? Inside meters, 1 m == 1;
>> inside mm, 1 mm == 1. Conversion functions could be nonclass
>> functions:
> I like the suggestion  I've been thinking along these lines lately. The
> only problem with having a nonclass conversion function is that there
> will be N^2 overloaded functions for N different units. That makes
> extensibility a problem : for example, if the library itself defines a
> few length units (pseudocode alert)
[...]
> Maybe the
> solution would be to have a default converter which looks something like
>
> template<typename model1,typename model2>
> convert<length_unit<model1>,length_unit<model2> >
> which uses SI as a common base and if the unit doesn't support
> conversion to/from SI, then the user would have to write the conversion
> function themselves... I'll have to think about this more...
You've pretty much got the support you need already: the toBase() method
in each UnitInfo.
You can define the "convert<>" function above in term to do (something
like  clearly this needs to fit into your template framework...)
return model1::toBase() / model2::toBase();
This is what I did in my (still not posted) version:
template<typename ValueType, typename Dimensionality,
typename NewUnitSystem, typename OldUnitSystem>
struct convert_units_helper
{
static ValueType scaling_factor()
{
return
(unit<Dimensionality, OldUnitSystem>::template scale<ValueType>()
/ unit<Dimensionality, NewUnitSystem>::template scale<ValueType>());
}
};
(unit<> is similar to your UnitInfo, "::scale<>" is like "::toBase<>" 
not exactly the same, but similar.)
The clean thing about this approach is that if you have 2 "small world"
systems, then scaling_factor() will evaluate (say):
1e30 / 1e27
So the scaling_factor applied to the *actual* value will be 1e3  which
is generally pretty accurate.
phil
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