iOS 5 Programming Cookbook

1.15. Allocating and Initializing Objects

Problem

You want to create an instance of a new object, but you don’t understand the difference between allocation and initialization and why you should have to both allocate and initialize an object before you can use it.

Solution

You must both allocate and initialize an object before using it. An object can be allocated using the alloc instance method. This class method will allocate memory to hold the object and its instance variables and methods. However, allocation leaves memory undefined. So in addition, each object must be initialized, which sets the values of its data. One initialization method must be the designated initializer, which is normally the initialization method with the most parameters. For instance, the initWithFrame: method is the designated initializer of objects of type UIView. Always allocate and initialize your objects, in that order, before using them.

When implementing a new object, do not override the alloc method. This method is declared in NSObject. Instead, override the init method and create custom initialization methods that handle required parameters for the specific object you are working on.

Discussion

An object that inherits from NSObject must be prepared for use in two steps:

Allocation

The allocation is done by invoking the alloc method, which is implemented in the NSObject class. This method creates the internal structure of a new object and sets all instance variables’ values to zero. After this step is ...