add forward declaration

add classes

CPlusPlus/Forward_Declaration/functions.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#ifndef _FUNCTIONS_H_
+#define _FUNCTIONS_H_
+
+void functionE(); // declared
+
+//void functionF() {} // declared & defined, linker error when included multiple times
+ 
+#endif // !_FUNCTIONS_H_