文档上记录是这样的
The Scope of Instance Variables
Toenforce the ability of an object to hide its data, the compilerlimits the scope of instance variables—that is, limits theirvisibility within the program.
为了强制一个对象隐藏其数据,编译器限制实例变量范围以限制其在程序中的可见性
But toprovide flexibility, it also lets you explicitly set the scope atfour levels. Each level is marked by a compilerdirective:
但是为了提供灵活性,苹果也让开发者显式设置范围(四选一)
Directive
M......
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