class Person { var residence: Residence? } class Residence { var numberOfRooms = 1 } let john = Person() // 链接可选residence?属性，如果residence存在则取回numberOfRooms的值 if let roomCount = john.residence?.numberOfRooms { print("John 的房间号为 \(roomCount)。") } else { print("不能查看房间号") }