add forward declaration examples

add classes

CPlusPlus/Forward_Declaration/functions.h

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