Re: Announce: Grace project site operational

["Grein, Christoph" <christoph.grein@eurocopter.com>]

From: "Martin Dowie" <martin.dowie@nospam.baesystems.com>
> "Hyman Rosen" <hyrosen@mail.com> wrote in message
> news:3CE15D0A.3050100@mail.com...
> [snip]
> > I first saw this idiom in _Scientific and Engineering C++_
> > by Bartion & Nackman.
> >
> > template<typename RepType, int Mass, int Distance, int Time>
> > struct Unit
> > {
> > RepType value;
> > explicit Unit(value) : value(value) { }
> > Unit operator+(Unit other)
> > { return Unit(value + other.value); }
> > Unit operator-(Unit other)
> > { return Unit(value - other.value); }
> > };
> [snip]
> 
> Didn't someone post an Ada units package similar to
> this a while back (> 1 year)?..

This is a theme that at least once a year re-appears. There is somewhere in 
Dabid Botton's AdaPower a paper by me about these physical types with a 
reference to a more detailed paper how my company handles this in several 
ebedded real-time avionics projects.

All "solutions" to this problem I've seen so far can be split into three 
methods:

1. Use subtypes with proper names like Meter, Kilogramm, Ohm just for
   documentation, i.e. no type checking.
2. Use strong types with separate types and operator overloading.
   This only works (except for broken exponents like 1/2 as in sqrt (Meter);
   yes, these occur quite often with rational values 1/2, 1/3, 3/2, 4/3 just
   to name some) for limited subsets of SI units (two or three).
3. Discrimiated types with dimensions carried as discriminants.
   This is the most general method which, if rational (not integer)
   arithmetic is applied to exponents, but is run-time intensive, so possibly
   not applicable to real-time.

There is a fourth method, preprocessors.
And a fifth, if you create a language that can handle dimensions.

My feeling is that method 1 should be the preferred one. two is out (expontial 
explosion of number of operations, rational exponents cannot be handled), 3 if 
execution time does not matter.

In theoretical phycial work, dimensionless forms of the equations are preferred 
after all.