autoreleasing on a thread

So basically, if you are running on OS X 10.9+ or iOS 7+, autoreleasing on a thread without a pool should not lead to a leak. This is not documented and is an internal implementation detail, so be careful relying on this as Apple could change it in a future OS. However, I don't see any reason why they would remove this feature as it is simple and only has benefits and no downsides, unless they completely re-write the way autorelease pools work or something.

The Objective-C runtime is open-source so you can read the source to see what's going on. The latest version of the runtime (646, which shipped with OS X 10.10 and iOS 8) does indeed add a pool if you perform an autorelease without a pool on the current thread. In NSObject.mm:

static __attribute__((noinline))

id *autoreleaseNoPage(id obj)

{

    // No pool in place.

    assert(!hotPage());

    if (obj != POOL_SENTINEL  &&  DebugMissingPools) {

        // We are pushing an object with no pool in place,

        // and no-pool debugging was requested by environment.

        _objc_inform("MISSING POOLS: Object %p of class %s "

                     "autoreleased with no pool in place - "

                     "just leaking - break on "

                     "objc_autoreleaseNoPool() to debug",

                     (void*)obj, object_getClassName(obj));

        objc_autoreleaseNoPool(obj);

        return nil;

    }

    // Install the first page.

    AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);

    setHotPage(page);

    // Push an autorelease pool boundary if it wasn't already requested.

    if (obj != POOL_SENTINEL) {

        page->add(POOL_SENTINEL);

    }

    // Push the requested object.

    return page->add(obj);

}

巴特西

autoreleasing on a thread

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