Re: [boost] physical-quantities (automatic conversions)

From: Matthias Schabel (boost_at_[hidden])
Date: 2004-01-09 10:29:52

> to avoid issue with "Unit1 operator + (Unit1 u1, Unit2 u2)"
> some kind of "expression templates" could be applied.
> operators like "+-*/" will return a compile time proxy object
> and in assign operator for unit it'll be converted in the type of this  
> unit.

The problem with this sort of approach as I see it is that there is  
still ambiguity as
to which unit system to convert to in an expression containing multiple  
mixed units :

Consider :

x meters + (y planck_length - z centimeters) ->

furlongs z = proxy<op<plus,meters,op<minus,planck_length,centimeters> >

Do we convert y and z to meters, x and z to planck_lengths, or x and y  
to centimeters before
the arithmetic and conversion to furlongs?  The outcomes can be  
different, as has been discussed earlier, due to precision issues.

------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
729 Arapeen Drive
Salt Lake City, UT  84108
801-587-9413 (work)
801-585-3592 (fax)
801-706-5760 (cell)
801-484-0811 (home)
mschabel at ucair med utah edu