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From: "kreckel at ginac dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/20758] New: operator-(const T&, const complex<T>&) vs operator-(const complex<T>&, const complex<T>&)
Date: Mon, 04 Apr 2005 21:38:00 -0000 [thread overview]
Message-ID: <20050404213802.20758.kreckel@ginac.de> (raw)
Here are three ways to express complex<double>(-4.,0.):
// complex<double> operator-<double>(const double&, const complex<double>&)
complex<double> a1 = 1.
- complex<double>(5.0, 0.0);
cout << a1 << endl; // (-4,-0)
// complex<double> operator-<double>(const complex<double>&, const
complex<double>&)
complex<double> a2 = complex<double>(1.0, 0.0)
- complex<double>(5.0, 0.0);
cout << a2 << endl; // (-4,0)
// complex<double> operator-<double>(const complex<double>&, const double&)
complex<double> a3 = complex<double>(1.0, 0.0)
- 5.0;
cout << a3 << endl; // (-4,0)
In the first version, the imaginary part gets a spurious minus sign. That is
quite disruptive when you consider that a1 could be the argument of a logarithm
because it then ends up on the wrong side of the branch cut.
While I am currently unable to track this to any standard, I would guess that
the resulting imaginary part should be governed by what 0.0-0.0 is. By analogy,
the first result would be wrong, unless its undefined behavior (which would be a
pity).
The problem is in libstdc++-v3/include/std/std_complex.h:366. I propose to
change the function body to not use unary operator- in the imaginary part. I'm
going to attach two analogous patches.
--
Summary: operator-(const T&, const complex<T>&) vs operator-
(const complex<T>&, const complex<T>&)
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: kreckel at ginac dot de
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20758
next reply other threads:[~2005-04-04 21:38 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-04-04 21:38 kreckel at ginac dot de [this message]
2005-04-04 21:39 ` [Bug libstdc++/20758] " kreckel at ginac dot de
2005-04-04 21:40 ` kreckel at ginac dot de
2005-04-04 21:42 ` kreckel at ginac dot de
2005-04-04 21:45 ` pinskia at gcc dot gnu dot org
2005-04-04 21:52 ` kreckel at ginac dot de
2005-04-04 23:18 ` pinskia at gcc dot gnu dot org
2005-04-04 23:37 ` pcarlini at suse dot de
2005-04-05 14:01 ` kreckel at ginac dot de
2005-04-05 14:36 ` pcarlini at suse dot de
2005-04-05 14:48 ` pcarlini at suse dot de
2005-04-05 15:29 ` pcarlini at suse dot de
2005-04-06 8:59 ` gdr at integrable-solutions dot net
2005-04-06 9:17 ` pcarlini at suse dot de
2005-04-06 9:27 ` pcarlini at suse dot de
2005-04-07 20:51 ` kreckel at ginac dot de
2005-04-07 21:17 ` pcarlini at suse dot de
2005-04-07 22:06 ` kreckel at ginac dot de
2005-04-07 22:19 ` pcarlini at suse dot de
2005-04-08 8:23 ` kreckel at ginac dot de
2005-04-08 9:55 ` pcarlini at suse dot de
2005-04-08 14:16 ` gdr at integrable-solutions dot net
2005-04-08 14:20 ` gdr at integrable-solutions dot net
2005-04-08 22:16 ` kreckel at ginac dot de
2005-04-24 22:49 ` kreckel at ginac dot de
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