C++Talk.NET

Stefan Große Pawig · Guest

Hello all,

TR1 introduced, in the C compatibility chapter, the function
fabs(complex<T>):

----- SNIP -----
8.1.1 Synopsis [tr.c99.cmplx.syn]

namespace std {
namespace tr1 {
[...]
template<class T> complex<T> fabs(const complex<T>& x);
} // namespace tr1
} // namespace std

[...]

8.1.8 Function fabs [tr.c99.cmplx.fabs]

1 Effects: Behaves the same as C99 function cabs, defined in
subclause 7.3.8.1.
----- SNIP -----

The current C++0X draft document (n2009.pdf) adopted this
definition in chapter 26.3.1 (under the comment // 26.3.7 values)
and 26.3.7/7.

But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document
n1124), the referenced subclause reads

----- SNIP -----
7.3.8.1 The cabs functions

Synopsis

1 #include <complex.h>
double cabs(double complex z);
float cabsf(float complex z);
long double cabsl(long double z);

Description

2 The cabs functions compute the complex absolute value (also called
norm, modulus, or magnitude) of z.

Returns

3 The cabs functions return the complex absolute value.
----- SNIP -----

Note that the return type of the cabs*() functions is not a complex
type. Thus, they are equivalent to the already well established
template<class T> T abs(const complex<T>& x);
(26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft
document n2009.pdf).

So either the return value of fabs() is specified wrongly, or fabs()
does not behave the same as C99's cabs*().

Proposed resolution:

This depends on the intention behind the introduction of fabs().

If the intention was to provide a /complex/ valued function that
calculates the magnitude of its argument, this should be
explicitly specified. In TR1, the categorization under "C
compatibility" is definitely wrong, since C99 does not provide
such a complex valued function.

Also, it remains questionable if such a complex valued function
is really needed, since complex<T> supports construction and
assignment from real valued arguments. There is no difference
in observable behaviour between

complex<double> x, y;
y = fabs(x);
complex<double> z(fabs(x));

and

complex<double> x, y;
y = abs(x);
complex<double> z(abs(x));

If on the other hand the intention was to provide the intended
functionality of C99, fabs() should be either declared deprecated
or (for C++0X) removed from the standard, since the functionality
is already provided by the corresponding overloads of abs().

Regards,
Stefan
--
Veni, vidi, VISA: I came, I saw, I did a little shopping

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Pete Becker · Guest

Stefan Große Pawig wrote:

Stefan Große Pawig · Guest

petebecker (AT) acm (DOT) org (Pete Becker) wrote:

Pete Becker · Guest

Stefan Große Pawig wrote: