Objective-C Fundamentals

58 CHAPTER 3 An introduction to objects
The core concept of inheritance is that a subclass becomes a more specialized version
of its superclass. Rather than describing all of the logic contained in the subclass from
scratch, only the differences in behavior from the superclass need to be specified.
Polymorphism is a related concept that enables you to treat any object, no matter
its class, as if it were typed as one of its superclasses. This works because subclasses can
only extend or modify the behavior of a class; they can’t remove functionality.
3.2 The missing data type: id
In chapter 2 we made one glaringly large omission when discussing the data types
available to represent data in an application. We didn’t cover how to store an object—
something rather important for a language starting with the word Objective!
In Objective-C a variable, which can represent an object, makes use of a data type
called id. For example:
id name = @"Christopher Fairbairn";
id is a special kind of data type in that it can store a reference to an object of any type.
Objective-C also enables you to be more specific about the type of object you expect to
store in a variable. For example, you would more commonly see the previous variable
declaration declared as follows:
NSString *name = @"Christopher Fairbairn";
Here the id data type is replaced by NSString *. Being more explicit about the type of
object you expect to store in the variable enables a number of helpful compile-time
checks. For starters, the compiler will produce an error if you attempt to assign or
store an object of another type to the variable, and a warning will be produced if you
attempt to send the object a message that, to the best of the compiler’s knowledge, it
can’t handle.
There’s no magic with id
You may think there’s some kind of magic occurring with the id data type. How can a
variable of type id store an object of any data type, and why don’t you need to specify
a * character after the data type, as you do with other class names such as NSString?
To answer these questions, declare a variable of type id and then double-click it while
holding down the Cmd key. You should see something similar to the following:
typedef struct objc_object { Class isa; } * id;
This is the declaration of the id data type. It’s another name (a type definition) for a
pointer to a struct called objc_object. The objc_object struct can be considered
as low-level “plumbing” in the Objective-C runtime. With a slight amount of hand waving,
you can consider it to be the same as NSObject. Because all objects in Objective-C
ultimately derive from NSObject, a variable of type id can store a pointer to
any object, no matter its type. Because the declaration of the id data type contains
a *, one isn’t required when it’s utilized in an application.