[boost] Re: math_constants parameterisation and return by ref

From: Deane Yang (deane_yang_at_[hidden])
Date: 2004-01-22 21:52:57

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;
}

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