Avoiding Redifinition using #pragma once

Lets write a very simple program where there is main.cpp that includes classA.h and classB.h. classA.h contains class A and classB.h contains class B.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Program to show how to avoid redifinition problems
#include <iostream>
#include "classA.h"
#include "classB.h"

using namespace std;

int main()
{
  A a;
  B b;
  a.a = 20; //some stuff not relevant here
  b.a = 40; //some stuff not relevant here
  return 0;
}

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//A very simple example class that does nothing
class A
{
public:
  int a;
  int b;
private:
  int c;
};

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//A very simple example class that does nothing
#include "classA.h"

class B :public A
{
private:
  int d;
};

When we try to compile this, the compiler complains as follows:

To get round this problem, in each of the include file we can use a #define as follows

#ifndef _CLASSA_H_

#define _CLASSA_H

//Some code

#endif //_CLASSA_H

In C++ we can also write #pragma once. Using #pragma once can increase compilation speed and the compilor may optimise the code as the use of pre-processor can be removed. So the class A in our program giving problem can now be modified as follows for the program to compile properly:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//A very simple example class that does nothing
#pragma once
class A
{
public:
  int a;
  int b;
private:
  int c;
};