If your deployment target is lower than iOS 6.0 or Mac OS X 10.8
You need to use dispatch_retain and dispatch_release on your queue. ARC does not manage them.
If your deployment target is iOS 6.0 or Mac OS X 10.8 or later
ARC will manage your queue for you. You do not need to (and cannot) use dispatch_retain or dispatch_release if ARC is enabled.
Starting in the iOS 6.0 SDK and the Mac OS X 10.8 SDK, every dispatch object (including a dispatch_queue_t) is also an Objective-C object. This is documented in the <os/object.h> header file:
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This means you can store your queue in an NSArray or NSDictionary, or in a property with one of the strong, weak, unsafe_unretained, assign, or retain attributes. It also means that if you refer to your queue from a block, the block will retain the queue automatically.
So if your deployment target is at least iOS 6.0 or Mac OS X 10.8, and you have ARC enabled, ARC will retain and release your queue, and the compiler will flag any attempt to use dispatch_retain or dispatch_release as an error.
If your deployment target is at least iOS 6.0 or Mac OS X 10.8, and you have ARC disabled, you must manually retain and release your queue, either by calling dispatch_retain and dispatch_release, or by sending the queue retain and release messages (like [queue retain] and [queue release]).
For compatibility with old codebases, you can prevent the compiler from seeing your queue as an Objective-C object by defining OS_OBJECT_USE_OBJC to 0. For example, you can put this in your .pch file (before any #import statements):
or you can add OS_OBJECT_USE_OBJC=0 as a preprocessor macro in your build settings. If you set OS_OBJECT_USE_OBJC to 0, ARC will not retain or release your queue for you, and you will have to do it yourself using dispatch_retain and dispatch_release.