[boost] Re: physical-quantities

From: Dan W. (danw_at_[hidden])
Date: 2004-01-08 16:19:08

Matthias Schabel wrote:
> The only problem with having a non-class conversion function is that  
> there will be N^2 overloaded functions for N different units.  That  
> makes extensibility a problem : for example, if the library itself  
> defines a few length units (pseudocode alert)
...snip...
> then if someone decides they want to use rods, they're going to have to  
> add 4*3-3*2 = 6 new functions.  Then the next person who needs fathoms  
> will have to add the 2(N-1) = 8 more functions, etc...  Maybe the  
> solution would be to have a default converter which looks something  like

I think you could have your cake and eat it too: You could have an 
internal absolute reference, as long as

A) It uses compile-time unlimited precision types (thinking of Plank's 
Lengths)
B) It is compile-time (mpl?) and generates whatever converters are 
needed, mindful of precision issues.
C) It is internal to the library, --i.e.: as a user, I'd never suspect 
its existence. Which means, I declare furlongs as const * cubits, and 
the conversion generator reads that and generates conversions to or from 
internal absolutes, as step 1, and later generates conversions to/from 
inches, if I *use* inches in the program, that's fine with me.