/**** Repeated Includes -- Example that will throw an error****/

/* Class A */

class A{
//...
};


/* Class B */

#include “classA.h”

class B{
//...
};


/* User */

#include “a.bad.h”
#include “b.bad.h” //you will get an error

int main() {return 0;}




/**** Include Guards -- How to fix it ****/

/* Class A Fixed*/

#ifndef A_
#define A_

class A{
//...
};

#endif


/* Class B Fixed */

#ifndef B_
#define B_

#include “classA.h”

class B{
//...
};

#endif


/* User */

#include “a.good.h”
#include “b.good.h” //no error this time

int main() {return 0;}