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  • The class that has no parent is at the top of the hierarchy and called the root class.


    You should take advantage of existing classes to use as your root class. NSObject is a common root class.

    Fraction: NSObject

    Fraction is the child or subclass.


    child class


    parent class

    Whenever a new class is defined it inherits certain things from the superclass including all non-private methods and instance variables.

    Instance variables that are accessed directly by a subclass must be declared in the interface section, and not in the implementation section. Instance variables declared or synthesized in the implementation section are PRIVATE instance variables and are not directly accessible by subclasses.

    sub-sub class


    alloc and init are inherited methods from NSObject


    Rules to choose a message (method) to apply to an object

    First, the class to which the object belongs is checked. Then the parent class, and then the next parent class, and so on. If you don't find the method a warning message is generated.

    Inheritance often is used to extend a class


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