Re: [boost] Re: physical-quantities

From: Matthias Schabel (boost_at_[hidden])
Date: 2004-01-09 13:38:43

> Hmmm. Not sure about that.  There is an implicit assumption there that  
> all
> quantities represented are "absolute" - that is wrong since it would
> require that either "fahrenheit(32) + fahrenheit(32) ==  
> fahrenheit(32)" or
> that such operations are not permitted.  Neither option is correct.

Good point - I guess in reality there should be a distinction made  
somewhere
between a value which represents, e.g., a temperature measurement and a
value which represents a temperature difference.  This is getting  
complicated...

> My gut feeling is that you need a convert_unit_relative and
> convert_unit_absolute - one that does what you used to do, and one that
> does what you want to do now (respectively).

I guess this sort of problem must have arisen in the date/time library  
as well
since a time interval is conceptually different than a time point.  It  
may be that
we need to have two distinct types in the units library, one relative  
and one
absolute, so that you could do things like

absolute = absolute + relative
relative = relative + relative

but not

absolute = absolute + absolute

Probably possible.  Then again...AUUUGGHHH...sorry...

------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
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mschabel at ucair med utah edu