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From: Hubert HOLIN <Hubert.Holin@meteo.fr>
Newsgroups: comp.std.c++
Subject: Template function overloading: must every candidate functions be 
 instanciated?
Date: Wed, 14 Feb 2001 19:34:33 GMT
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Xref: supernews.google.com comp.std.c++:3637

Somewhere in the E.U., le 14/02/2001

	Considering the following complete code, my compiler (MetroWerks
CodeWarrior Pro 6) indicates a compile error (illegal operand) on line
15, that's the "if	(abs(x) <= ::std::numeric_limits<T>::epsilon())" line
in the first appearing definition of sinc_pi.

	The message error in itself is reasonable, as numeric_limits is not
instanciated for ::std::complex<float>, and the compiler is trying to
instanciate "complex<float> sinc_pi<complex<float> >(const
complex<float> x)".

	However, I wonder if it is necessary to actually instanciate this
function. It is necessary to *consider* this function for the overload
resolution, but the second function should be preferred as it is more
specialized (though the differing return type makes that statement
slightly dubious, at least to me), so actually instanciating the first
function seems unnecessary.

	In short, is the following code legal, or not:


 --- 8>< --------------------------- ><8 --- 
#include <cmath>
#include <limits>
#include <complex>


/* This is the "Sinus Cardinal" of index Pi*/

template<typename T>
inline T	sinc_pi(const T x)
{
	using	::std::abs;
	using	::std::sin;
	
	if	(abs(x) <= ::std::numeric_limits<T>::epsilon())
	{
		return(T(1));
	}
	else
	{
		return(sin(x)/x);
	}
}

template<template<typename T> class U>
inline T	sinc_pi(const U<T> x)
{
	using	::std::abs;
	using	::std::sin;
	
	if	((abs(x) <= ::std::numeric_limits<T>::epsilon())
	{
		return(T(1));
	}
	else
	{
		return(sin(x)/x);
	}
}


/***					***/
/***	The Problem		***/
/***					***/

int	main()

{
	::std::complex<float>	c(1,2);
	
	sinc_pi(c);
}
 --- 8>< --------------------------- ><8 --- 

		Hubert Holin
		Hubert.Holin@Bigfoot.com
		Hubert.Holin@meteo.fr

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