4.6 Include guard added

Author: akshaybabloo

4_Preprocessors/4_6_Include_Guard/in1.h

@@ -0,0 +1,11 @@
+//
+//  in1.h
+//  CPP-Notes
+//
+//  Created by Akshay Raj Gollahalli on 30/05/16.
+//  Copyright © 2016 Akshay Raj Gollahalli. All rights reserved.
+//
+
+#include "in2.h" // We are including in2.h here
+
+int b = 20;

4_Preprocessors/4_6_Include_Guard/in2.h

@@ -0,0 +1,14 @@
+//
+//  in2.h
+//  CPP-Notes
+//
+//  Created by Akshay Raj Gollahalli on 30/05/16.
+//  Copyright © 2016 Akshay Raj Gollahalli. All rights reserved.
+//
+
+#ifndef in2_h // Check if in2.h is not defined, if so
+#define in2_h // define it.
+
+int a = 10;
+
+#endif /* in2_h */

4_Preprocessors/4_6_Include_Guard/main.cpp

@@ -0,0 +1,20 @@
+//
+//  CPP-Notes
+//
+//  Created by Akshay Raj Gollahalli on 23/05/16.
+//  Copyright © 2016 Akshay Raj Gollahalli. All rights reserved.
+//
+
+#include <cstdio>
+#include "in1.h"
+#include "in2.h"
+using namespace std;
+
+
+int main(int argc, char ** argv) {
+    
+    printf("a = %d, b = %d\n",a,b);
+    
+    return 0;
+}
+

README.md

@@ -36,6 +36,7 @@ C++ Notes
 	- [4.4 Macro expansion](#44-macro-expansion)
 	- [4.5 Problems with Macro's](#45-problems-with-macros)
 	- [4.6 Line continuation](#46-line-continuation)
+	- [4.6 Include guard](#46-include-guard)
 
 <!-- /TOC -->
 
@@ -866,3 +867,16 @@ If you want to use complex macros you can use `line continuation` by add `\` at
                     printf("%d\n",i++); \
                 } while (i < 3);
 ```
+
+### 4.6 Include guard
+
+See [4_6_Include_Guard](https://github.com/akshaybabloo/CPP-Notes/tree/master/4_Preprocessors/4_6_Include_Guard)
+
+There might be a situation where you might have to define a header file in another header file and when called in a C++ file you might include both header files. When you compile this you will have a `Build fail`, to over come this we have to include something called as `Include guard`. It looks something like this
+
+```cpp
+#ifndef _HEADERNAME_H
+#define _HEADERNAME_H
+...
+#endif
+```