摘要： NSObject类定义了copy方法，但这只是一个convinence method for classes who adopt NSCopying协议。如果某个类没有实现copyWithZone:，那调用copy方法会抛出异常。NSObject类的方法mutableCopy对应于NSMutableCopying protocol的方法mutableCopyWithZone:NSObject does not itself support the NSCopying protocol. Subclasses must support the protocol and implement the 阅读全文