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);
}