Why tack a protocol of NSObject to a protocol implementation

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When you declare a variable like

 id<MyProtocol> var;

the Objective-C compiler knows only about the methods in MyProtocol and will thus produce a warning if you try to call any of the NSObject methods, such as -retain/-release, on that instance. Thus, Cocoa defines an NSObject protocol that mirrors the NSObject class and instance methods. By declaring that MyProtocol implements the NSObject protocol, you give the compiler a hint that all of the NSObject methods will be implemented by an instance that implements MyProtocol.

Why is all this necessary? Objective-C allows objects to descend from any root class. In Cocoa, NSObject is the most common, but not the only root class. NSProxy is also a root class, for example. Therefore an instance of type id does not necessarily inherit NSObject‘s methods.

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