Re: [boost] units: review

From: Eric Lemings (lemings_at_[hidden])
Date: 2007-03-29 19:08:38

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Lewis Hyatt
> Sent: Thursday, March 29, 2007 4:36 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] units: review
> 
> Eric Lemings wrote:
> 
> >> quantity<SI::length> l1(2.0 * SI::meters);
> >> quantity<SI::length> l2(2.0 * Astro::parsecs); //for 
> illustration only
> > 
> > I would express that differently:
> > 
> > quantity<SI::length> l1 (quantity<SI::meter>(2.0));
> > quantity<SI::length> l2 (quantity<Astro::parsecs>(2.0)); //for
> > illustration only
> > 
> > And I could be way off base here but the latter form would 
> appear more
> > familiar/acceptable to most C++ programmers.
> 
> I think it is perfectly natural to think of units as having their own 
> algebra, and so the multiplication notation makes sense. How 
> would you 
> do something like this:
> 
> quantity<velocity> v(2.0 * meters/second);
>
> in your method? I think you'll find that this one is preferable.

quantity<velocity> v1 (quantity<meter>(2) / quantity<second>(1));
//or
quantity<meter> m(2);
quantity<second> s(1);
quantity<velocity> v2(m/s);

The problem with the "multiplier" usage is it uses a unit name (plural,
singular, whatever) for both type information and rvalue expressions.

quantity<second[s]> t1 (2.0 * second[s]);

This usage may be preferable to scientists and engineers but not C++
programmers, I'll gar-on-tee.

Eric.