Re: [boost] formal review request : mcs::units

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-02-09 12:21:55

> 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

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Matthias Schabel, Ph.D.
Assistant Professor, Department of Radiology
Utah Center for Advanced Imaging Research
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matthias dot schabel at hsc dot utah dot edu
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