Re: [boost] Compile-time power of a run-time base

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-02-04 13:48:30

AMDG

Bruno Lalande wrote:
> Yes I think Boost.Math is the right place for this too. And Boost.Unit will
> be able to take it there instead of having it confined in a "units::details"
> or something, this way everybody can benefit of this useful thing.
>
> I can write tests and docs for you, as well as study the different
> performance / optimization aspects with some other compilers. As I've never
> done this before, I'll have to read the different guidelines describing the
> way in which such contributions have to be made, so it can take a few days.
> But I think we're not in a hurry.
>
> Steven, could you tell me if some aspects of the implementation you made for
> Boost.Unit are better than mine? This would permit to be sure to have the
> best implementation in Boost.Math.
>   

Your implementation seems to be faster in the few tests I ran on msvc 9.0.

It would be nice if boost::units::pow could be completely replaced by 
this utility,
so here are the issues that would need to the covered:

* The result type is not necessarily going to be the same as the arguments
   so it would need to use Boost.Typeof or some such.
* Rational exponents need to work, so I would need a way to specialize
   the function to work with boost::units::static_rational.  I would expect
   the customization mechanism to look something like this:

struct default_value_tag;

template<class T>
struct pow_value_tag {
    typedef default_value_tag tag;
};

template<class Exp>
struct pow_exponent_tag {
    typedef typename Exp::tag type;
};

template<class ValueTag, class ExponentTag>
struct pow_impl;

template<class Value, class Exponent>
struct pow {
    typedef typename pow_impl<
        typename pow_value_tag<Value>::type,
        typename pow_exponent_tag<Exponent>::type
    >::template apply<Value, Exponent> impl;
    typedef typename impl::type type;
    static type call(const Value& v) {
        return(impl::call(v));
    }
};

In Christ,
Steven Watanabe