More on Variables and Data Types(Chapter 10 of Programming in Objective-C 2.0)

　　You can put four directives in front of your instance variables when they are declared in the interface section, to more precisely control their scope:

　　@protected—Methods defined in the class and any subclasses can directly access the instance variables that follow.This is the default case.

　　@private—Methods defined in the class can directly access the instance variables that follow, but subclasses cannot.

　　@public—Methods defined in the class and any other classes or modules can directly access the instance variables that follow.

　　@package—For 64-bit images, the instance variable can be accessed anywhere within the image that implements the class.