this is a gcc file, you can download it and learn its usetility, for further detail please look at t

				// Build don't link:				// 				// Copyright (C) 2001 Free Software Foundation, Inc.				// Contributed by Nathan Sidwell 11 Apr 2001 								// Bug 1944. We failed to calculate nearly emptiness properly, and				// lost primary too.								struct A1 {};				struct A2 {};								struct B1 : virtual A1 {};				struct B2 : virtual A2 {};								struct C1 : virtual B2 {};				struct C2 : virtual B2 {};								struct D1 : virtual C1, virtual C2 {};				struct D2 : virtual C2, virtual B1 {};								struct E : virtual D1, virtual D2 {};