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From: Martin Sebor <sebor@roguewave.com>
Newsgroups: comp.std.c++
Subject: Re: Template function overloading: must every candidate functions be 
 instanciated?
Date: Thu, 15 Feb 2001 23:21:28 GMT
Organization: Rogue Wave Software, Inc.
Approved: Fergus Henderson <fjh@cs.mu.oz.au>, moderator of comp.std.c++
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Hubert HOLIN wrote:
> 
> 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.

I believe the program you give below is ill-formed. Specifically, the part

> template<template<typename T> class U>
> inline T        sinc_pi(const U<T> x)
> {

is wrong, since, if I'm reading 14.1, p13 correctly, the scope of T ends at the
end of U.

In any case, you wouldn't want to (or couldn't) return T from the function but
rather "U<T>" (but you can't do that since T is not in scope). I don't see why
you'd want to use template template parameters here, but if you must this would
compile (I have no idea whether it does what you want, though :)

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


> 
>         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.

Candidate function templates are not instantiated during overload resolution.
Only the best viable function may eventually be instantiated.

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

Nope.

Regards
Martin

> 
>  --- 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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