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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).


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