[boost] Re: physical-quantities

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2004-01-08 12:19:03

 From what I've seen so far, I like your approach very much. Although I 
have to admit I haven't looked at it (or anyone else's proposals) very 
close. Anyway, the shown example syntax is IMHO as good as it can get in 
C++.

Matthias Schabel wrote:
> I don't really know about compile time reduction - a smart compiler  
> will undoubtedly use a hash of some sort to reduce names into integers  
> or something more manageable...  As far as code-writing time goes, I  
> guess I spend so much more time thinking about implementation and  
> algorithms than actually typing code that the long names are not an  
> issue.  I also have a personal preference for completely unambiguous  
> naming and, in this case, most users won't see this verbose stuff as it  
> is more internal to the library.  On the other hand, I'm not  
> particularly strongly attached to this particular choice of names...

I share your preference for long, unambiguous names. And a decent 
editor/IDE should have some support for them (e.g. (x)emacs' 
dabbrev-expand). Also, my experience tells me that the correctness of 
the program and catching stupid programmer errors is ways more important 
than the compile time.

> This snippet is carrying some baggage from an earlier version which was  
> not specifically tied to the SI model; i.e. it was written to deal with  
> arbitrary unit systems.  A more concise version with the existing  
> library would be :
> 
> template<class Y>
> typename SI<Y>::quantity
> idealGasLaw(typename SI<Y>::pressure P,
>             typename SI<Y>::volume V,
>             typename SI<Y>::temperature T)
> {
>     return (P*V/(8.314*(_joules/(_kelvin*_mole))*T));
> }

This looks very nice so far, but I have one question: Why is <Y> 
attached to SI instead of:

template<class Y>
typename SI::quantity<Y>
idealGasLaw(typename SI::pressure<Y> P,
             typename SI::volume<Y> V,
             typename SI::temperature<Y> T)
{
     return (P*V/(8.314*(_joules/(_kelvin*_mole))*T));
}

as it is AFAICS not part of the SI-model to define the number-type. Or 
was this just a typo?

Another question: Have you looked at my constant library? I think it 
could make a good companion of your library, reducing the above to

     return P * V / ( boost::chemistry::G * T );

with a properly defined boost::chemistry::G and now being truly generic 
as it would even works for types Y than don't mix with doubles. If you 
are interested in exploring this or have questions about the constant 
library, drop me a private mail.

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial solutions & technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de