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Memory Management: Object Ownership

Quartz uses the Core Foundation memory management model, in which objects are reference counted. When created, Core Foundation objects start out with a reference count of 1. You can increment the reference count by calling a function to retain the object, and decrement the reference count by calling a function to release the object. When the reference count is decremented to 0, the object is freed. This allows objects to safely share references to other objects.

There are a few simple rules to keep in mind:

• If you create or copy an object, you own it, and therefore you must release it. That is, in general, if you obtain an object from a function with the words “Create” or “Copy” in its name, you must release the object when you’re done with it. Otherwise, a memory leak results.

• If you obtain an object from a function that does not contain the words “Create” or “Copy” in its name, you do not own a reference to the object, and you must not release it. The object will be released by its owner at some point in the future.

• If you do not own an object and you need to keep it around, you must retain it and release it when you’re done with it. You use the Quartz 2D functions specific to an object to retain and release that object. For example, if you create a CGColorspace object, you use the functions CGColorSpaceRetain and CGColorSpaceRelease to retain and release the object as needed. You can also use the Core Foundation functions CFRetain and CFRelease, but you must be careful not to pass NULL to these functions.

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