Re: [boost] Quantity libraries: SIunits

From: Dean Foster (foster_at_[hidden])
Date: 2001-09-13 06:12:15

>
> X-eGroups-From: wb_at_[hidden] (Walter Brown)
>
> Adding new quantities, too, is useful.  However, you are suggesting
> adding a new quantity that requires adding a new "dimension" first.  I
> understand what you are proposing, but the International System of
> Units does not provide any guidance in this direction. 
>

I certainly like the adherance to standards.  Fruther the SI standards
have been fairly well thought out.  So it does make sense to have a
pure SI-units library.  I guess I'm argueing that it also makes sense
to have a user-defined-units library.  For example, the code that I've
been working on needs the following units:

        CPU-time (a clock that stops when paged out)
        SI-time (this would be a SI)
        byte    (unit of information--I think this is non-SI)
        dollar  (unit money)

I'll then use derived units like dollar/byte second, dollar/CPU, etc.
But, I never want to add CPU-time to SI-time.  They are measuring
different things and any code that wants to add them together is
mostlikely incorrect.  If it is correct, then it should at least
require a cast to draw attention to the fact that something weird is
being done.  (Or more accurately multiplying by a conversion factor
that might be CPU-time/SI-time, but on a 4 processor parallel machine
the right conversion might be 4 CPU-time/SI-time.)

It might very well be that most applications fall either in the domain
of hard core physics where SI does it all or fall well outside physics
that so that few of the SI units would be useful.  This would argue for
creating two entirely seperate libraries.

So the question in my mind isn't whether these two programming styles
are both important (I beleive they are) but whether we want to have
two libraries or only one library.

dean

=============================================================================
Dean Foster                                                   dean_at_[hidden]
Statistics, Wharton, U. Penn                                     215 898 8233
Philadelphia PA 19104-6302                 http://diskworld.wharton.upenn.edu