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

From: Martin Bonner (Martin.Bonner_at_[hidden])
Date: 2008-02-05 05:58:47

----Original Message----
From: boost-bounces_at_[hidden]
[mailto:boost-bounces_at_[hidden]] On Behalf Of John Maddock Sent:
05 February 2008 10:26 To: boost_at_[hidden]
Subject: Re: [boost] [Boost] Compile-time power of a run-time base

> Bruno Lalande wrote:
>>> * 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.
>> 
>> 
>> Yep, I had already thought about this improvement.
> 
> 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.  

>>> * 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

> I'm a bit concerned about this interface: my guess is that 90% or
> more of users would just want integer exponents, and these should
> be very easy to use, preferably simply:
> 
> pow<2>(val)
> 
> Hmmm, I wonder if we can define an overload such that:
> 
> pow<2,3>(val)
> 
> is also valid?

... 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.
-- 
Martin Bonner
Senior Software Engineer/Team Leader
PI SHURLOK LTD
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