Why are #ifndef and #define used in C++ header files?

#ifndef <token>
/* code */
#else
/* code to include if the token is defined */
#endif

#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement. #ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file.

#ifndef _INCL_GUARD
#define _INCL_GUARD
#endif

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

This prevent from the multiple inclusion of same header file multiple time.

#ifndef __COMMON_H__
#define __COMMON_H__
//header file content
#endif

Suppose you have included this header file in multiple files. So first time __COMMON_H__ is not defined, it will get defined and header file included.

Next time __COMMON_H__ is defined, so it will not include again.