1. 工程项目模板类型的更改
Xcode4时代 Xcode4.2新模板
Navigation-based Application ------->Master-Detail Application => iphone
Splite View-based Application ------->Master-Detail Application => ipad
OpenGL ES Application --------------->OpenGL Game
Tab Bar Application --------------->Tabbed Application
Utility Application --------------->Utility Application
View-based Application --------------->Single View Application
Window-based Application------------>Empty Application
2. 新增一编译级特性——Automatic Reference Counting
这似乎是Xcode4.2也增加了类似其他面向对象语言的GC（垃圾回收器）的概念。不知道我的理解是否正确？但在实践中若是iphone应用编程或导入以前的工程项目最好还是禁用ARC，（我们自己勤快点养成释放清理对象的习惯。）方法为：Project Info -> Apple LLVM compiler 3.0 - Language -> Objective-C Automatic Referencing Counting， 设置为NO即可。
否则会对release，retainCount，retain等内存相关操作在编译时出现红色警告：