vb.net barcode maker 9: Memory Management in Objective-C

Generator QR Code ISO/IEC18004 in Objective-C 9: Memory Management

CHAPTER 9: Memory Management
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Memory management is a hard problem. Cocoa s solution is rather elegant but does take some time to wrap your mind around. Even programmers with decades of experience have problems when first encountering this material, so don t worry if it leaves your head spinning for awhile. If you know that your programs will only be run on Leopard or later, you can take advantage of Objective-C 2.0 s garbage collection, which we ll discuss at the end of this chapter. We won t feel sad if you skip to the end, really. If you want to run on older versions of Mac OS X or you re doing iPhone development, you will want to read the whole chapter.
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Object Life Cycle
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Just like the birds and the bees out here in the real world, objects inside a program have a life cycle. They re born (via an alloc or a new); they live (receive messages and do stuff ), make friends (via composition and arguments to methods), and eventually die (get freed) when their lives are over. When that happens, their raw materials (memory) are recycled and used for the next generation.
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Reference Counting
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Now, it s pretty obvious when an object is born, and we ve talked a lot about how to use an object, but how do we know when an object s useful life is over Cocoa uses a technique known as reference counting, also sometimes called retain counting. Every object has an integer associated with it, known as its reference count or retain count. When some chunk of code is interested in an object, the code increases the object s retain count, saying, I am interested in this object. When that code is done with the object, it decreases the retain count, indicating that it has lost interest in that object. When the retain count goes to 0, nobody cares about the object anymore (poor object!), so it is destroyed and its memory is returned to the system for reuse. When an object is created via alloc or new, or via a copy message (which makes a copy of the receiving object), the object s retain count is set to 1. To increase its retain count, send the object a retain message. To decrease its retain count, send the object a release message. When an object is about to be destroyed because its retain count has reached 0, Objective-C will automatically send the object a dealloc message. You can override dealloc in your objects. Do this to release any related resources you might have allocated. Don t ever call dealloc directly. You can rely on Objective-C to invoke your dealloc method when it s time to kill your object. To find out the current retain count, send the retainCount message. Here are the signatures for retain, release and retainCount:
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CHAPTER 9: Memory Management
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- (id) retain; - (void) release; - (unsigned) retainCount; Retain returns an id. That way, you can chain a retain call with other message sends, incre-
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menting its retain count and then asking it to do some work. For instance, [[car retain]
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setTire: tire atIndex: 2]; asks car to bump up its retain count and perform the setTire action.
The first project in this chapter is RetainCount1, located in the 09.01 RetainCount-1 project folder. This program creates an object (RetainTracker) that calls NSLog() when it s initialized and when it gets deallocated:
@interface RetainTracker : NSObject @end // RetainTracker
@implementation RetainTracker - (id) init { if (self = [super init]) { NSLog (@"init: Retain count of %d.", [self retainCount]); } return (self); } // init
- (void) dealloc { NSLog (@"dealloc called. Bye Bye."); [super dealloc]; } // dealloc @end // RetainTracker
The init method follows the standard Cocoa idiom for object initialization, which we ll explore in the next chapter. As we mentioned earlier, the dealloc message is sent (and, as a result, the dealloc method called) automatically when an object s retain count reaches 0. Our versions of init and dealloc use NSLog() to write out a message saying that they were called.
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