[boost] Re: math_constants parameterisation and return by ref

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2004-01-23 03:41:02

Deane Yang wrote:
> Daniel Frey wrote:
> 
>> The problem with constants is generic programming. Consider:
>>
>> template< typename T > T area( const T& r ) {
>>    return pi * r * r;
>> }
>>
>> Now think of different T's, mixed arithmetics, loss of precision, no 
>> conversion from double to T (e.g. boost::rational, Roguewave's 
>> RWDecimal<>, ...). You need a pi in the above example which is truly 
>> generic. *That* is what I'm working on, the syntax pi vs. pi() is just 
>> syntactic sugar (although I strongly prefer pi over pi()). More on 
>> that when I finished the restructuring due to the unit libraries :)
> 
> You can do that? That would be absolutely awesome.
> But are you really trying to coordinate with the unit libraries?
> If so, shouldn't the function be something like
> 
> template< typename length_units, typename area_units>
> area_units area( const length_units& r )
> {
>    return pi * r * r;
> }

If you'd like to use a unit library, then you are right. My example was 
just a rationale for the constant library for the "normal" programmer 
that currently often doesn't know about unit-libraries. But this just 
gave me another small idea which is IMHO possible to implement: Both 
implementations can share a single 'pi' when it's defined correctly :)

> Can you really implement "pi" so that it is able to look inside "r"
> and figure out what numeric type "pi" should be?

This is exactly what I'm working on at the moment, and using __typeof__ 
it is possible.

I'm also trying to make the constant library independent of the unit 
library, that means I'll try to post an example for Matthias' YANL (the 
idealGasLaw-function he posted), but I will not depend on YANL. It is my 
hope that the constant library (which doesn't define the constants 
itself, it's just a framework for others to define their constants in 
the right way) is then used by the different unit libraries around here. 
I think I can show the complete idealGasLaw-example after this weekend.

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial solutions & technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de