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

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-02-19 16:45:30

Steven,

> template<class DimensionMap, class Tag, class System>
> struct  
> base_unit_converter<Tag,heterogeneous_system<DimensionMap>,System> :
>     base_unit_converter<Tag, typename mpl::at<DimensionMap,  
> Tag>::type,System> {
> };

I actually think we can just leave the base_unit_converter template  
alone since this deals
with a converting values of one fundamental dimension between two  
systems. Then we
can just specialize conversion_helper for homogeneous systems to be  
the current
implementation and have the default specialization deal with  
heterogeneous systems:

// current implementation for homogeneous systems
template<class S1,
                 class S2,
                 class Dim1,
                 class Y>
class conversion_helper< quantity<unit<homogeneous_system<S1>,Dim1>,Y>,
                                                 quantity<unit<homogeneous_system<S2>,Dim1>,Y> >

// generic implementation for heterogeneous systems
template<class System1,class Dim1,
                 class System2,class Dim2,
                 class Y>
class conversion_helper< quantity<unit<System1,Dim1>,Y>,
                                                 quantity<unit<System2,Dim2>,Y> >

The actual implementation of the latter will probably be the hardest  
part...
I've plugged in specializations for homogeneous_system<S> into the  
code base and
all examples still work correctly. Once we have a functioning system  
that properly
sorts heterogeneous units, we should just have to write  
specializations for

struct multiply_typeof_helper< unit< heterogeneous_system<S>,Dim1>,
                                                           unit< heterogeneous_system<S>,Dim2> >

and

struct divide_typeof_helper< unit< heterogeneous_system<S>,Dim1>,
                                                         unit< heterogeneous_system<S>,Dim2> >

unary_plus_typeof_helper, unary_minus_typeof_helper, add_typeof_helper,
subtract_typeof_helper, power_typeof_helper, and root_typeof_helper  
all stay
the same as far as I can see...

I'm starting to feel optimistic that we'll be able to get something  
very flexible
without adding too much complexity...

Matthias