Why it's valid to include a header file twice in c++?

Question

#include \"DLLDefines.h\"
#include \"DLLDefines.h\"

The above actually passed compilation, but why?

Answer 1

Probably you have some #define in DLLDefines.h around your code that prevents it from being included twice.

#ifndef DLLDEFINES_H
#define DLLDEFINES_H
// your code
#endif