Objective-C Fundamentals

Autorelease pools
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Ideally, you’d have a way to signal to Objective-C that you’re ready for ownership of
an object to be released, but instead of relinquishing ownership immediately, you prefer
it to be performed at some point in the future, once the caller has had a chance to
claim ownership for itself. The answer to this scenario is called an autorelease pool.
To use an autorelease pool, you simply send the object an autorelease message
instead of the more immediate release message. This technique is demonstrated by
the following version of the CreateMessageForPerson method:
- (NSString *)CreateMessageForPerson:(NSString *)name
{
NSString *msg = [[NSString alloc] initWithFormat:@"Hello, %@", name];
[msg autorelease];
return msg;
}
With the string added to the autorelease pool, you’ve relinquished ownership of the
string, yet it’s safe to continue referring to and accessing it until some point in the future
when it’ll be sent a more traditional release message on your behalf. But what exactly
is an autorelease pool, and when will the string be deallocated?
9.3.1 What is an autorelease pool?
An autorelease pool is simply an instance of the NSAutoreleasePool class. As your application
runs, ownership of objects can be transferred to the most current autorelease
pool. When an object is added to an autorelease pool, it takes over ownership of the
object and sends the object a release message only when the pool itself is deallocated.
This can be handy when you want to create a bunch of temporary objects. Instead
of manually keeping track of each retain and release message you send, you can
simply use the objects, safe in the knowledge that they will all eventually be released.
9.3.2 Adding objects to the autorelease pool
Adding an object to the current autorelease pool is easy. You simply send the object an
autorelease message instead of a release:
[myObject autorelease];
This causes the current autorelease pool to retain the object after relinquishing the
current owner of its ownership responsibilities.
9.3.3 Creating a new autorelease pool
In general, it’s possible to use the services of an autorelease pool without having to
explicitly create one. This is because Cocoa Touch creates pools behind the scenes for
your use. But it’s important to at least know when and how these pools are created so
you know when objects will eventually be deallocated and how much “garbage” may
accumulate beforehand.
To create a new autorelease pool, you simply create an instance of the NSAutorelease-
Pool class:
NSAutoreleasePool *myPool = [[NSAutoreleasePool alloc] init];