Mac OS X Leopard - ARCAism

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CHAPTER 26 MAC OS X DEVELOPMENT: OBJECTIVE-C
NOTE This section discusses the traditional Objective-C model for manual memory management.
Although it is still valid and useful information, Leopard introduces an alternative:
automatic memory management, also known as garbage collection. You can read more about
garbage collection in the “Objective-C 2.0” section.
To ameliorate this risk, Objective-C uses a system called retain counting. Retain counting is
extremely simple, at least in concept. Every object that subclasses NSObject has an instance variable
called retainCount. When an object is created by alloc or copy, retainCount is initialized to 1.
Anyone interested in the object sends it a retain message, which increments retainCount. Anyone
no longer interested in the object sends it a release message. When retainCount reaches 0, the
object is freed, as shown in Figure 26-2.
Figure 26-1. The risk of malloc/free in object- Figure 26-2. Retain counting lets objects be
oriented programming shared safely.
If everyone remembers to retain and release at the appropriate times, this system works perfectly,
except for one little problem. What happens if you create an object and then pass it to
someone else, as illustrated in Figure 26-3? The caller can’t retain the object until after you
return it. If you release it before you return it, the retain count will drop to zero, and the object
will be freed. If you return it without releasing it, the retain count will never drop to zero, and
the memory will leak!
The solution to this problem is to let a third party retain the object, as shown in Figure 26-4.
That way, the object will not be freed until both the caller and the third party release it. This is
accomplished by sending the object an autorelease message. The system will then place the
object into an autorelease pool. The autorelease pool will keep track of objects on request and
then send them the release message at the bottom of the event loop.
If everyone has released an object before the autorelease pool has released it, the end result
is the same. The object will be freed when the autorelease pool releases it. However, if a lot of
objects are created and released, as in a long loop, the programmer may need to create an inner
autorelease pool and drain it manually. This is easily done; autorelease pools are objects that are
instances of NSAutoreleasePool.