Re: [boost] [units]

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-03-05 19:01:25

Engineering approximations are often derived by regression to  
empirical functional forms,
which can result in all kinds of dimensional weirdness, including  
floating point powers. In
principle these could be accommodated within a dimensional analysis  
framework. In
practice, floating point powers are impossible for a compile-time  
library. On the other hand,
we do support rational powers and, for engineering approximations,  
you might as well
use a rational approximation to the powers since it is unlikely that  
they will be exactly
equal to some irrational value...

Matthias

>> There's just plain going to be times when you have to break out of  
>> the
>> dim/unit analysis model.  For instance, in fluid flow analysis,  
>> what I
>> do, you often have empirical functions...guesswork...it is not at all
>> uncommon to take a quantity to some variable power that might  
>> itself be
>> a quantity.  There's no way to enforce dimensions on this.
>
> Is this really right? Are you sure there isn't a constant lurking  
> around
> that resolves all the dimensions properly? Surely, these formulas have
> to be adjusted appropriately if you use different units.
>
> Can you provide a concrete example?