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

Important If you use Automatic Reference Counting (ARC), you cannot use autorelease pools directly. Instead, you use
@autoreleasepool blocks instead. 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.