Re: [boost] [units] Crazy idea/question

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-02-15 16:49:32

>> 1) The target unit may only have one instance of each tag (no
>> repeated tags with different
>> systems). Otherwise, the conversion becomes ambiguous.
>
> If the source has only one instance of each tag
> than it should be easy too.

But in general we should assume that constructs like this are legit:

quantity<SI::volume>	V(1.0*SI::meters*imperial::feet*CGS::centimeters);

where the system of the result type defines the conversions

imperial::feet->SI::meters
CGS::centimeters->SI::meters

to get the conversion factor...this is unambiguous in all cases, AFAICS.

> A more sophisticated data structure might be
> appropriate
>
> dim<
>     length_tag,
>     mpl::list<
>         mpl::pair<SI::system, static_rational<2> >,
>         mpl::pair<CGS::system, static_rational<1> >

I guess the problem is that the optimal data structure for compile-time
algebra won't be the same as the optimal data structure for conversions.
For the former, you really just have three steps:

1) append two sequences
2) sort by unit system/tag type
3) reduce duplicate tags

For the latter, you really want to

1) sort source type by tag_type
2) iterate over systems, converting to destination system
3) reduce duplicate tags

Seems like these two are at odds, but I may just not be thinking about
it right. Also, making the data structure more complicated would, I  
imagine,
pose a problem for compile times again...

Matthias