Why are #ifndef and #define used in c++ header files

Question

I have been seeing code like this usually in the start of header files

#ifndef HEADERFILE_H
#define HEADERFILE_H

and at the end of the file is

#endif

I am confused about the purpose of this ..?

Answer 1

Those are called Include guards.

Once the header is included, it checks if a unique value (in this case HEADERFILE_H) is defined. Then if it's not defined, it defines it and continues to the rest of the page.

When the code is included again, the first ifndef fails, resulting in a blank file.

That prevent double declaration of any identifiers such as types, enums and static variables.