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I’ve noticed that a lot of Apple’s interfaces use @private before their instance variable declarations. Is there a good design reason for this, and is it something I should do?
Private instance variables are used to separate interface from implementation. In Objective-C, since the class declaration must show all of the instance variables, there needs to be a way to prevent subclasses from accessing the ones that are part of the internal implementation. Otherwise, other programmers could write code that depends on those internal variables, which would make it impossible for the class designer to make changes to the class internals without breaking existing code.
Looking at it another way, the instance variables that are not marked private are part of a contract with the subclass programmer, whereas the ones marked private are not.
This means that instance variables should usually be marked private so that they can only be accessed through their accessor methods, if at all. Otherwise, someone could easily write a subclass of your class, and simply create an accessor to make any instance variable public.