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From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2007-03-28 15:07:41

Steven Watanabe wrote:

> The confusion could be on my part. I was thinking
> that the combo box would allow arbitrary data to
> be associated with the each item. If this is not
> the case then you need either a map or a switch
> statement regardless of whether the units are compile
> time or runtime.

Well, not arbitrary but some data that is meaningful to the code using
it that is not displayed to the user. In my case I was thinking a unit
type that exists solely for providing conversions. Not always is that a
multiplication. For instance gage pressures adds an extra value on top
of any conversion when converted from an absolute. There are also
cases, that though may be rather questionable, in which two disparate
dimensions are used for the same kind of thing. For instance, a user
can often request that pressures are reported in a "hydrostatic"
pressure, which is a length. A hydrostatic unit might contain
information necessary to do the dimensional conversion to be used in a
pressure quantity. For this reason the unit type cannot be a primitive
if the library is to encompass this use, which I think it should.

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.

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. How often is one going to use multiple "systems"? The
problem of writing expressions in one set of units that may be different
than the rest of the system is easily accomplished with the use of
static constants. I believe that the cases when a static system is
better are actually very small and it is for that reason that I don't
believe this library is a good candidate for boost inclusion since the
object of boost is to provide generic and commonly useful libraries.
The dimensional analysis part is very generic and useful but the static
only unit part detracts from this usefulness.

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