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

From: John Maddock (john_at_[hidden])
Date: 2008-02-05 11:56:54

Martin Bonner wrote:
>> The only use case I can think of is where the argument is an integer
>> rather than a real number (and the result is therefore a double),
>
> boost.units:
> pow<3>( length )
> is going to have units of volume.

Ah yes, forgot that.  My gut feeling is that Boost.Math should concentrate 
on the um... math: provide a nice simple implementation that's reasonably 
efficient too.  And libraries like Boost.Units can overload it for 
dimentioned-quantities, and use the Boost.Math version for the internal 
calculation.  Does this make sense?

> ... and I /believe/ that boost.units only needs half powers.  So
> (modulu units)
>
>      template<class T1, class T2, int N>
> T2 pow<N,2>(T1 val) { return sqrt(pow<N>(val)); }
>
> should do it.
>
> ... in fact, I suspect that boost.math pow should /only/ offer the
> integral powers, and boost.units should wrap it to add the rational
> support.

I guess the question is can we evaluate half-powers more efficiently than 
just using pow?  Maybe, but it's not so obvious to me as it is for smallish 
integer powers.

John.