https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121615
Bug ID: 121615
Summary: Function abs(complex<int>) does not compute correct
results
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: chris21k at gmx dot de
Target Milestone: ---
$ cat show_abs_bug.cpp
#include <complex>
#include <iostream>
int main() {
std::cout << std::abs(std::complex<int>(3, -4)) << std::endl; //
expected sqrt(3^2 + (-4)^2) = 5 and not 4
}
$ g++ show_abs_bug.cpp -o show_abs_bug
$ ./show_abs_bug
4
$ less /usr/include/c++/complex # snippet
// 26.2.7/3 abs(__z): Returns the magnitude of __z.
template<typename _Tp>
inline _Tp
__complex_abs(const complex<_Tp>& __z)
{
_Tp __x = __z.real();
_Tp __y = __z.imag();
const _Tp __s = std::max(abs(__x), abs(__y));
if (__s == _Tp()) // well ...
return __s;
__x /= __s;
__y /= __s;
return __s * sqrt(__x * __x + __y * __y);
}
$ g++ --version
g++ (Debian 14.2.0-19) 14.2.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
cat /etc/issue
Debian GNU/Linux forky/sid \n \l
$ uname -a
Linux betty 6.12.38+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.38-1
(2025-07-16) x86_64 GNU/Linux
(The called) above abs implementation in header file <complex> imho seems to
only be correct for complex<"floating point"> but not for complex<"integral"> .
Either there is a bug in the detection which implementation should be called or
the above implementation should more general.
Btw. same behaviour with gcc 12.2.0 in Debian bookworm. Unfortunately, I have
no test with a newer gcc, sorry for that.
Many thanks,
Chris
