What's the difference between declaring a variable “id” and “NSObject *”?

Question

In Objective-C, what\'s the difference between declaring a variable id versus declaring it NSObject *?

Answer 1

id means "an object", NSObject * means "an instance of NSObject or one of its subclasses". There are objects in Objective-C which are not NSObjects (the ones you'll meet in Cocoa at the moment are NSProxy, Protocol and Class). If some code expects an object of a particular class, declaring that helps the compiler check that you're using it properly. If you really can take "any object" - for instance you are declaring a delegate and will test all method sends with respondsToSelector: calls - you can use an id.

Another way to declare an object variable is like "id ", which means "any object which implements the NSObject protocol.