Re: [boost] Boost Units library preview

From: Eric Lemings (lemings_at_[hidden])
Date: 2006-09-01 18:53:06

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Ben Strasser
> Sent: Friday, September 01, 2006 9:15 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] Boost Units library preview
> 
> I'm having a serious problem with facets and if I don't find 
> a solution for it facets are definately out of the game.
> 
> Several unit types can describe the same unit. For example 
> unit<quantity<length, 1, mass, 1> > is the same as 
> unit<quantity<mass, 1, length, 1> > but when it comes to 
> types they are different.

Why do you use the word 'unit' in these examples?  I don't see
any units in there.  A unit is a standard of measure.  A meter
is a unit.  Mass and length are dimensions.

> The unit code can cope with this. facets on the other hand 
> requier an exact type to be able to use unit specific facets.
> 
> I only see 3 possible solutions:
> 
> 1) Add a facet for each permutation of the base quantities (= 
> length & time_ here). This is only acceptable for small 
> quantities because the number of combinations grows too fast. 
> Also I don't like the idea of spamming the locale objects full.
> 
> 2) Make the quantity lists sortable and only look up the 
> sorted versions in the locale objects. The problem is that 
> for this I need some smaller-as meta function to define the 
> order. Unfortuately I have no idea on how to generate a 
> default one (typeid unfortuately only return runtime objects). 

If you look at the proposed Boost Units library, the base
dimensions are implemented in such a way (with a rank) that
allows them to be normalized (e.g. sorted).

Eric.