Includes the C++ standard library header <complex>.
The C standard library <complex.h> header isn't included by <ccomplex>, because it's effectively replaced by the C++ overloads in <complex> and <cmath>. That makes the <ccomplex> header redundant. The <complex.h> header is deprecated in C++. The <ccomplex> header is deprecated in C++17 and removed in the draft C++20 standard.
clog, which is declared in <complex.h>, isn't defined in the
std namespace because of potential conflicts with the
clog that's declared in <iostream>.