including a header file twice in c++

Question

What happens if I include iostream or any other header file twice in my file? I know the compiler does not throw error.

Will the code gets added twice

Answer 1

It simply gets skipped over, due to preprocessor code along the following lines:

#ifndef MY_HEADER_H
#define MY_HEADER_H



#endif

So if you include twice, then MY_HEADER_H is already defined and everything between the #ifndef and #endif is skipped by the preprocessor.