Re: Announce: Grace project site operational

[Hyman Rosen <hyrosen@mail.com>]

Stephen Leake wrote:
> The C++ template solution apparently computes and checks the Mass,
> Distance, and Time values at _compile_ time, so there is no runtime
> overhead. There is, in effect, the same combinatoric explosion of
> types (actually operators in this case), but it is handled
> automatically by the compiler, thru implicit instantiation (is that
> the right term?). And only those operators that are actually used are
> ever declared, and the _user_ only has to write the templates, not
> the instantiations.

Correct, in all respects. Furthermore, because the operations
are so simple and can be declared inline, the explosion is only
within the compiler (and perhaps the symbol table). The actual
code will contain only simple arithmetic, just as if there were
no units at all.

In a real implementation, there would also be operators to
allow multiplication and division by scalars, and scaled units
are just handled by multiplication:

typedef Unit<double, 1, 0, 0> Mass;
const Mass gram = Mass(1);
const Mass kilogram = 1000 * gram;
const Mass pound = 2.2 * kilogram;

Other posters have been mentioning fractional units. The C++
approach could accomodate this either by having the base unit
be the least common multiple of all the roots (i.e., if you
need meter^1/2 and meter^1/3 you could work in meter^1/6 and
use powers of that) or by using rationals for the powers (so
instead of Unit<Mass> you can have Unit<MassN,MassD>; the
arithmetic can all still be done in the compiler, not at
runtime).