From: Steven Watanabe (steven_at_[hidden])
Date: 2007-03-29 21:03:52
AMDG
Noah Roberts <roberts.noah <at> gmail.com> writes:
>
> Steven Watanabe wrote:
> > Do you think you can provide me with a
> > concrete case that I cannot solve easily using the
> > current library?
>
> Since any problem can be solved under almost any circumstance, no.
Key word: easily.
Never mind. What you say below answers my question
> <snip>
>
> There is one use case that we have in our products that is not covered
> by that. User defined units. These are specified as some conversion to
> an arbitrary base we set up.
Ah. So to do such conversions you need:
template<class Quantity>
class unit_converter {
typedef typename get_unit<Quantity>::type Unit;
typedef typename get_dimensions<Unit>::type Dimensions;
typedef typename Quantity::value_type value_type;
public:
typedef Quantity result_type;
typedef const value_type& argument_type;
typedef result_type function_type(const value_type&);
template<class System>
unit_converter(const unit<Dimensions, System>&) :
converter(static_cast<function_type*>(&convert_impl<OtherSystem>)) {}
unit_converter(const value_type& factor) :
converter(lambda::bind(from_value(), lambda::_1 * factor)) {}
unit_converter(const value_type& factor, const value_type& offset) :
converter(lambda::bind(from_value(), lambda::_1 * factor + offset)) {}
result_type operator()(const value_type& v) const {
return(converter(v));
}
private:
boost::function<function_type> converter;
template<class OtherSystem>
static result_type convert_impl(const value_type& v) {
return(result_type(v * unit<Dimensions, OtherSystem>()));
}
struct from_value {
template<class T>
struct sig { typedef result_type type; };
result_type operator()(const value_type& v) const {
return(result_type::from_value(v));
}
}
};
Adding the reverse conversion ~doubles the size. Again
if this doesn't fit you needs it may take even more code.
>
> Your idea of a allowing the unit override might solve may of my
> objections. I would have to review that feature as it came. When I was
> looking at this library it did not seem at all straight forward to do so.
Allowing it is pretty straight forward. I admit
that it probably doesn't look like it unless you
know the internals well.
In Christ,
Steven Watanabe