NSAutoreleasePool class is used to support Cocoa’s reference-counted memory management system. An autorelease pool stores objects that are sent a
release message when the pool itself is drained.
- iOS 2.0+
- macOS 10.0+
- tvOS 9.0+
- watchOS 2.0+
In a reference-counted environment (as opposed to one which uses garbage collection), an
NSAutoreleasePool object contains objects that have received an
autorelease message and when drained it sends a
release message to each of those objects. Thus, sending
autorelease instead of
release to an object extends the lifetime of that object at least until the pool itself is drained (it may be longer if the object is subsequently retained). An object can be put into the same pool several times, in which case it receives a
release message for each time it was put into the pool.
In a reference counted environment, Cocoa expects there to be an autorelease pool always available. If a pool is not available, autoreleased objects do not get released and you leak memory. In this situation, your program will typically log suitable warning messages.
The Application Kit creates an autorelease pool on the main thread at the beginning of every cycle of the event loop, and drains it at the end, thereby releasing any autoreleased objects generated while processing an event. If you use the Application Kit, you therefore typically don’t have to create your own pools. If your application creates a lot of temporary autoreleased objects within the event loop, however, it may be beneficial to create “local” autorelease pools to help to minimize the peak memory footprint.
You create an
NSAutoreleasePool object with the usual
init messages and dispose of it with
release—to understand the difference, see Garbage Collection). Since you cannot retain an autorelease pool (or autorelease it—see
autorelease), draining a pool ultimately has the effect of deallocating it. You should always drain an autorelease pool in the same context (invocation of a method or function, or body of a loop) that it was created. See Using Autorelease Pool Blocks for more details.
Each thread (including the main thread) maintains its own stack of
NSAutoreleasePool objects (see Threads). As new pools are created, they get added to the top of the stack. When pools are deallocated, they are removed from the stack. Autoreleased objects are placed into the top autorelease pool for the current thread. When a thread terminates, it automatically drains all of the autorelease pools associated with itself.
If you are making Cocoa calls outside of the Application Kit’s main thread—for example if you create a Foundation-only application or if you detach a thread—you need to create your own autorelease pool.
If your application or thread is long-lived and potentially generates a lot of autoreleased objects, you should periodically drain and create autorelease pools (like the Application Kit does on the main thread); otherwise, autoreleased objects accumulate and your memory footprint grows. If, however, your detached thread does not make Cocoa calls, you do not need to create an autorelease pool.
In a garbage-collected environment, there is no need for autorelease pools. You may, however, write a framework that is designed to work in both a garbage-collected and reference-counted environment. In this case, you can use autorelease pools to hint to the collector that collection may be appropriate. In a garbage-collected environment, sending a
drain message to a pool triggers garbage collection if necessary;
release, however, is a no-op. In a reference-counted environment,
drain has the same effect as
release. Typically, therefore, you should use
drain instead of