Memory Management: “reference counting” overview

Cocoa implements its ownership policy through a mechanism called “reference counting” or “retain counting.” When you create an object, it has a retain count of 1. When you send an object a retain message, its retain count is increased by 1. When you send an object a release message, its retain count is decreased by 1 (autorelease causes the retain count to be decremented in the future).

When its retain count drops to 0, an object’s memory is reclaimed—in Cocoa terminology it is “freed” or “deallocated.” When an object is deallocated, its dealloc method is invoked automatically. The role of the dealloc method is to free the object's own memory, and dispose of any resources it holds, including its object instance variables.

If your class has object instance variables, you must implement a dealloc method that releases them, and then invokes super's implementation. For example, if the Thingamajig class had name and sprockets instance variables, you would implement its dealloc method as follows:

- (void)dealloc

{

[sprockets release];

[name release];

[super dealloc];

}

You should never invoke another object’s dealloc method directly.

http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmObjectOwnership.html#//apple_ref/doc/uid/20000043-BEHDEDDB