Avoid multiple inclusions!

a header file may be included more than once!

this may cause errors!

the column on the right shows how to avoid this.

// this is file one.h #include "some.h" void one() { ... }	#ifndef ONE_H #define ONE_H <program goes here> #endif
// this is file some.h void some() { ... }	#ifndef SOME_H #define SOME_H <program goes here> #endif
// this is main.cpp #include "some.h" #include "one.h" void mainprog() { ... }