Objective-C Fundamentals

100 CHAPTER 5 Creating classes
Sometimes subtle differences have the most profound effect
In Objective-C code it’s common to see the superclass explicitly declared as NSObject.
If you’re a Java or C# developer, this may seem redundant, because in those
languages omitting the superclass causes the compiler to automatically inherit your
class from a predefined superclass (java.lang.Object or System.Object, respectively).
Objective-C is similar to these languages in that it supports only single inheritance
but, unlike those languages, it allows for multiple root classes. Not every class
must ultimately inherit from NSObject.
If you don’t explicitly provide a superclass, Objective-C declares your class as a new
root class. In other words, the class will sit beside NSObject in the class hierarchy
rather than below it. While this is possible, it generally isn’t desirable because it
means your new class won’t inherit any of the standard functionality typically associated
with an Objective-C object. Helpful methods such as isEqual: and memory
management such as alloc, init, and dealloc are all implemented by NSObject.
the class associates with each object created, while the second section declares any
methods and properties the class wishes to allow clients to access.
Before discussing how objects can provide logic to manipulate or work with the
instance data they store, let’s look at how you store data in a class.
5.2.1 Instance variables (ivars)
One of the first steps in defining a new class is to determine the type of data it needs
to store. For the CTRentalProperty class, you want to store data similar to that which
you previously stored in the RentalProperty structure. At a minimum, you’d like to
store the following data for each rental property:
■
Rental price per week (float)
■ Address of property (NSString *)
■
Property type (enum)
In the context of a class, such fields are called instance variables (or ivars, for short).
Instance variables are specified inside a set of curly braces in a class’s @interface declaration
and appear just like ordinary variable declarations: you provide the data type
and name of each variable and terminate each statement with a semicolon. For example,
you could declare the instance variables of the CTRentalProperty class as follows:
float rentalPrice;
NSString *address;
PropertyType propertyType;
Each time you create a new object of the CTRentalProperty class, a unique set of
these variables will be allocated to store the details of this particular instance. Therefore,
if you create multiple CTRentalProperty instances to represent a group of rental
properties, each will be able to store its own unique rentalPrice, address, and
propertyType values.