add forward declaration examples

add classes

CPlusPlus/Forward_Declaration/main.cpp

@@ -0,0 +1,20 @@
+#include "functions.h"
+
+void functionB() {} // declared & defined
+void functionC(); // declare
+void functionD(); // declare
+
+int main()
+{
+	//functionA(); // compiler error, because function is declared & defined after this call
+	functionB(); // no error
+	//functionC(); // linker error, because function is declared, but not defined
+	functionD(); // no error
+	functionE(); // no error
+
+	return 0;
+}
+
+void functionA() {} // declare & defined
+
+void functionD() {} // defined