<ccomplex>
Includes the STL header <complex>, which effectively includes the Standard C library header <complex.h> and adds the associated names to the std namespace.
#include <ccomplex>
Remarks
Including this header ensures that the names declared using external linkage in the Standard C library header are declared in the std namespace.
The name clog, which is declared in <complex.h>, is not defined in the std namespace because of potential conflicts with the clog that is declared in <iostream>.