Inherits From: none(NSObject is the root class)
Conforms To: NSObject
Declared In: Foundation/NSObject.h
NSObject is the root class of all ordinary Objective C inheritance hierarchies; it has no superclass. Its interface derives from two sources: the methods it declares directly and those declared in the NSObject protocol. Its interface is divided in this way so that objects inheriting from other root classes (notably NSProxy) can stand in for ordinary objects without having to inherit from NSObject. The following discussion makes no distinction between the methods declared in this class and those declared in the NSObject protocol.
From NSObject, other classes inherit a basic interface to the run-time system for the Objective C language. It's through NSObject that instances of all classes obtain their ability to behave as objects. Among other things, the NSObject class provides inheriting classes with a framework for creating, initializing, deallocating, comparing, and archiving objects, for performing methods selected at run-time, for querying an object about its methods and its position in the inheritance hierarchy, and for forwarding messages to other objects. For example, to ask an object what class it belongs to, you'd send it a class message. To find out whether it implements a particular method, you'd send it a respondsToSelector: message
The NSObject class is an abstract class; programs use instances of classes that inherit from NSObject, but never of NSObject itself.
Initializing an Object to Its Class
Every object is connected to the run-time system through its isa instance variable, inherited from the NSObject class. isa identifies the object's class; it points to a structure that's compiled from the class definition. Through isa, an object can find whatever information it needs at run timesuch as its place in the inheritance hierarchy, the size and structure of its instance variables, and the location of the method implementations it can perform in response to messages.
Because all ordinary objects inherit directly or indirectly from the NSObject class, they all have this variable. The defining characteristic of an object is that its first instance variable is an isa pointer to a class structure.
The installation of the class structurethe initialization of isais one of the responsibilities of the alloc and allocWithZone: methods, the same methods that create (allocate memory for) new instances of a class. In other words, class initialization is part of the process of creating an object; it's not left to the methods, such as init, that initialize individual objects with their particular characteristics.
Instance and Class Methods
Every object requires an interface to the run-time system, whether it's an instance object or a class object. For example, it should be possible to ask either an instance or a class whether it can respond to a particular message. So that this won't mean implementing every NSObject method twice, once as an instance method and again as a class method, the run-time system treats methods defined in the root class in a special way:
Instance methods defined in the root class can be performed both by instances
and by class objects.
A class object has access to class methodsthose defined in the class and those inherited from the classes above it in the inheritance hierarchybut generally not to instance methods. However, the run-time system gives all class objects access to the instance methods defined in the root class. Any class object can perform any root instance method, provided it doesn't have a class method with the same name.
For example, a class object could be sent messages to perform NSObject's respondsToSelector: and perform:withObject: instance methods:
SEL method = @selector(riskAll:);
if ( [MyClass respondsToSelector:method] )
[MyClass perform:method withObject:self];
When a class object receives a message, the run-time system looks first at the receiver's set of class methods. If it fails to find a class method that can respond to the message, it looks at the set of instance methods defined in the root class. If the root class has an instance method that can respond (as NSObject does for respondsToSelector: and perform:withObject:), the run-time system uses that implementation and the message succeeds.
Note that the only instance methods available to a class object are those defined in the root class. If MyClass in the example above had reimplemented either respondsToSelector: or perform:withObject:, those new versions of the methods would be available only to instances. The class object for MyClass could perform only the versions defined in the NSObject class. (Of course, if MyClass had implemented respondsToSelector: or perform:withObject: as class methods rather than instance methods, the class would perform those new versions.)
Initializing the Class