iOS 5 Programming Cookbook

1.17. Moving From Manual Reference Counting to Automatic Reference Counting

Problem

You want to learn about Automatic Reference Counting, Apple’s new Compiler solution to solving the headache that programmers had to deal with when working with objects and memory management in Objective-C.

Note

Automatic Reference Counting eliminates many of the manual reference counting issues that ultimately resulted in iOS apps that would crash here and there, and would be very unstable when deployed on user devices. ARC removes this headache by leaving most of the memory management complexity to the compiler.

Solution

Study the new storage attributes introduced with the latest LLVM compiler: strong, weak, and unsafe_unretained.

Discussion

In the latest LLVM complier, to use Automatic Reference Counting (ARC), we will need to deal with storage that is strong, weak, or unsafe and unretained. Any object under ARC is managed with one of these storage attributes. Here is a short explanation for each one:

strong

An object of this type is automatically retained at runtime and will be valid until the end of its scope, where it will automatically be released. For those familiar with Objective-C’s traditional way of memory management, this keyword is similar to the retain keyword.

weak

This is zeroing weak referencing. If a variable is defined with this keyword, when the object to which this variable points gets deallocated, this value will get set to nil. For instance, if you have a strong string property and a ...