Objective-C Fundamentals

A whirlwind tour of object-oriented programming concepts
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3.1.3 What are classes?
A class is a specification, or blueprint, designed to describe the structure of one or
more objects in a system that share a similar purpose. Classes are the most basic form
of encapsulation in Objective-C: they combine a small amount of data with a set of relevant
functions to manipulate or interact with that data.
Once a class is defined, its name becomes a new data type, meaning that you can
declare a variable to be of that type. The class is like a factory line, rolling out cookie
after cookie on demand. When instantiated, each newly created object has its own
copy of the data and methods defined by the class. By convention, class names begin
with a capital letter to differentiate them from method and instance variable names,
which typically start with a lowercase letter.
3.1.4 Inheritance and polymorphism
An advantage of object-oriented programming is the ability to reuse existing classes
time and time again. Not only can you create additional objects with little effort, but
one class can build on the foundations of another. This technique, called inheritance, is
similar to a family tree. One class in the system inherits, or derives, from another.
Inheritance has two major benefits:
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Code reuse—A subclass inherits all the data and logic defined by its superclass
(ancestor). This avoids duplicating identical code in the definition of similar
classes.
Specialization—A subclass can append additional data or logic and/or override
existing behavior provided by the superclass.
Class clusters
While researching Objective-C you may come across the concept of class clusters.
These are an example of inheritance and polymorphism. In a class cluster, a superclass
is documented, while a number of subclasses are purposely left undocumented
(as private, implementation-specific details).
As an example, virtually all Objective-C tutorials, including this book, discuss using
the NSString class to store text. It may surprise you that in most cases there will
never be an object of type NSString created in your application.
Instead, when you request a new NSString object, one of a number of subclasses
with names such as NSCFString is created in its place. Because these subclasses
inherit from NSString, they can be used in its place. As the old saying goes, “If it
walks like a duck, quacks like a duck, and swims like a duck, it’s probably a duck.”
NSString uses these “hidden” subclasses to allow itself to optimize memory and
resource usage based on the specifics of each string generated.
The next time you’re in the Xcode debugger, hover the mouse over a variable of type
NSString. In the data tip that appears, you’ll probably see the object’s data type
listed as NSCFString. This is a class cluster in action.