NSAutoreleasePool & thread

https://developer.apple.com/documentation/foundation/nsautoreleasepool

An object that supports Cocoa’s reference-counted memory management system.

An autorelease pool stores objects that are sent a

release

message when the pool itself is drained.

Important

If you use Automatic Reference Counting (ARC), you cannot use autorelease pools directly. Instead, you use

@autoreleasepool

blocks. For example, in place of:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// Code benefitting from a local autorelease pool.
[pool release];

you would write:

@autoreleasepool {
// Code benefitting from a local autorelease pool.
}

@autoreleasepool

blocks are more efficient than using an instance of

NSAutoreleasePool

directly; you can also use them even if you do not use ARC.

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

alloc

and

init

messages and dispose of it with

drain

(or

release

—to understand the difference, see Garbage Collection). Since you cannot retain an autorelease pool (or autorelease it—see

retain

and

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.

Threads

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.

Note

If you are creating secondary threads using the POSIX thread APIs instead of

NSThread

objects, you cannot use Cocoa, including

NSAutoreleasePool

, unless Cocoa is in multithreading mode. Cocoa enters multithreading mode only after detaching its first

NSThread

object. To use Cocoa on secondary POSIX threads, your application must first detach at least one

NSThread

object, which can immediately exit. You can test whether Cocoa is in multithreading mode with the

NSThread

class method

isMultiThreaded

.

Garbage Collection

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

release

.