http://www.apple.com.cn/developer/iphone/library/documentation/UserExperience/Conceptual/MemoryMgmt/MemoryMgmt.html
http://www.apple.com.cn/developer/mac/library/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingIvarsAndTypes.html#/apple_ref/doc/uid/20001284
