From: Steven Watanabe (steven_at_[hidden])
Date: 2007-03-28 15:27:55
AMDG
Noah Roberts <roberts.noah <at> gmail.com> writes:
> <snip>
>
> In my case I was thinking a unit
> type that exists solely for providing conversions. Not always is that a
> multiplication.
>
> <snip>
Ok. Good point. You can use type erasure to get this effect
and be able to use the static conversions at runtime.
template<class Quantity, class InputQuantity>
Quantity quantity_converter(const typename Quantity::value_type& v) {
return(quantity_cast<Quantity>(InputQuantity::from_value(v)));
}
> Also, your idea has problems when you account for rounding. Rounding is
> often done before you display data to a user. In cases when this data
> can come from a calculation or from the user it is hard to decide when
> to round and when not. If you apply rounding and conversions on both
> input and output and add rounding into the mix you can have a user
> entered value that gets reported back to the user as something else.
> This is not acceptable in many cases.
I may be wrong, but in order to avoid any excess loss
of precision you have to store a set of all the base
units and track the actual unit at runtime involving
a merge with every multiplyor divide. This kind of
overhead is not always acceptable.
> At any rate, there are ways to solve the whole thing regardless. The
> question is, what is the use of a library that only does static
> conversions.
As has been stated many times conversions are not
the primary point of the library.
> How often is one going to use multiple "systems"?
Hopefully not very often.
In Christ,
Steven Watanabe