If exponents of 4 are the highest you've seen, shouldn't you consider the size of (2*3*5*7*11*13*17)^4, which requires 76 bits? Also, I may want to extend the list - some people have requested a "money" unit, for example, which would be assigned the value 19 in your system. Then I can't even represent all powers of 3. I suspect this idea, while clever, does not cover the problem domain sufficiently well to be superior to lists.
I agree that this is a clever idea; I imagine that implementing it would not be too difficult, just to see how much it speeds compilation relative to the optimized list-based code that Steven put together. Essentially, as James points out above, we would be trading generality for performance. I would certainly happy to test out a replacement for dimension.hpp based on this approach. One concern I have is how to ensure that each fundamental unit has a unique prime number associated with it. I guess the current system has the same problem, though counting integers is clearly easier than counting primes. That actually raises a question I had a while ago: Is there some method to count specializations of a template class? That is, if I have template<long N> struct ordinal { typedef typename boost::mpl::int_<N> value; }; I want to be able to do something like this (warning not C++): struct tag1 : public next_ordinal::type { }; struct tag2 : public next_ordinal::type { }; struct tag3 : public next_ordinal::type { }; etc... where next_ordinal::type resolves to ordinal<1>, ordinal<2>, and ordinal<3> on sequential calls. Anyone have any ideas? This would simplify and make safer the process of defining user-defined unit systems. Matthias ---------------------------------------------------------------- Matthias Schabel, Ph.D. Assistant Professor, Department of Radiology 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) matthias dot schabel at hsc dot utah dot edu ----------------------------------------------------------------