add forward declaration examples

add class & inheritance examples

CPlusPlus/forward_declaration/functions.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#ifndef _FUNCTIONS_H_
+#define _FUNCTIONS_H_
+
+void functionE(); // declaration
+//void functionF() {} // defined & declared, linker error when included multiple times
+
+#endif // _FUNCTIONS_H_