Cocoa in a Nutshell

Creating Object Instances

One of the principal functions of a class object is to serve as a factory for creating new instances. When new objects are created, memory is allocated and its instance variables are initialized. This is accomplished by using the alloc method, defined by the NSObject class, as follows:

Song song = [song alloc];

The alloc class method dynamically allocates memory, sets the isa variable to a pointer to the class’s class object, sets all other variables to 0, and then returns the new object instance. This takes care of the system level tasks that need to be performed when an object is created, but doesn’t allow the object to properly initialize itself. To give an opportunity for object-specific initialization, the NSObject class provides the init instance method. To fully create an instance of the Song class, use the following code:

Song song = [[song alloc] init];

The init method can be overridden in a subclass to assign defaults to instance variables and to take care of other tasks that need to be performed before an object is used.

Tip

You can call the alloc and init methods by using separate lines of code. However, since object allocation and initialization are interlinked, calling both methods with one line of code is good practice.

When you override the init method, the superclass’s init method (or designated initializer, as covered in the next section) should always be called to ensure that the superclass is initialized properly. Initialization methods ...