Boost logo

Boost :

From: Paul A. Bristow (boost_at_[hidden])
Date: 2003-10-11 10:57:08

I hate to be negative about your proposal (unlike many Boosters who seem to take
great delight in shooting/shouting anything down) but having glanced at the code
examples, I really don't think that this is the right way to go. I still think
that everything should be held internally in SI units - now that double has
enough range to cover almost everyones needs. I'm worried about conversion
loss - your first example warns of the danger. And I am worried about the
expansion to other all the types of units - many are unecessary (feet even!) but
widely used.

But we definitely still need a good system for Quantity units etc.


Paul A Bristow, Prizet Farmhouse, Kendal, Cumbria, LA8 8AB UK
+44 1539 561830 Mobile +44 7714 33 02 04

| -----Original Message-----
| From: boost-bounces_at_[hidden]
| [mailto:boost-bounces_at_[hidden]]On Behalf Of Andy Little
| Sent: Friday, October 03, 2003 4:55 PM
| To: boost_at_[hidden]
| Subject: [boost] Physical Quantities revisited
| Hi, I am a Newbie to Boost :-)
| I am currently working on an upgrade to a
| point-and-click small wind turbine rotor blade design app,
| which involves various physics.
| The original did its math in terms of inbuilt types ,
| but significant time is spent looking up (say)
| whether a length is in metres or millimetres
| or an angle in degrees or radians.
| I have also had requests from users for ability to work
| in (say) feet and inches etc rather than s.i. units.
| I have had a look at various physical quantities libraries
| including the "quantities" library at boost,
| and SIunits at
| but they have an odd syntax.
| ie both do something like:
| length mylength = 20 * metres(); // here length is a type
| whereas what would seem to be more useful would be:
| metres myLength(20); // here metres is the type
| //*1 see footnote on explicit ctor
| The critical difference here is that the type is the
| unit of dimension, rather than the dimension itself.
| As a programmer if I want to know the units
| of a variable they are encoded in the type
| and if I can't decipher them I can do a conversion:
| metres mylength = metres(length_with_unknown_units);
| // ok... whatever units length_with_unknown_units was ,
| // mylength is its value in metres.
| I have spent some time creating a physical_quantities class template,
| with this approach, and have put the source code and
| a demo programme (and sample output) on my web site at:
| I would be interested to hear any views
| as to whether this approach is worth pursuing, or suggestions or
| improvements.
| ( Alternatively there may be a library that does this that I havent found
| ... ? )
| note: I have tried to answer some obvious questions and criticisms
| in the Potential FAQ on the page.
| ---
| *1 / this example is actually an error if coded as:
| metres myLength =1;
| in my version of physical_quantities
| because it violates dimensional analysis.
| _______________________________________________
| Unsubscribe & other changes:

Boost list run by bdawes at, gregod at, cpdaniel at, john at