#including a file more than once during the same compilation session will most likely cause the compiler to issue error messages since definitions and declarations of the file are repeated. However, in many cases, the same file may be #included in different source file that are compiled together:
#include "Time.h" //file Time.h itself contains #include "chrono.h" directive#include "Date.h" //file date.h also contains #include "chrono.h" directivevoid main() {Date d;Time t; }In order to avoid compile time errors, every header file should have an #include guard to ensure it is scanned only once in a single compilation session:
//file Chrono.h#ifndef CHRONO_H //is this file #included the first time during this compilation?#define CHRONO_H true//declarations, namespace definitions, typedefs and any other stuff goes here:class Chrono { /*_*/ };#endifIf CHRONO_H is defined, the compiler ignores the contents of Chrono.h between #ifndef and #endif. The first time Chrono.h is scanned during compilation, its contents are read and CHRONO_H is defined. Any subsequent inclusion thereafter is harmless since the file contents will be ignored. Bear in mind that all standard headers have #include guards.
