Answer by eug for NSArray of weak references (__unsafe_unretained) to objects under ARC

I've just faced with same problem and found that my before-ARC solution works after converting with ARC as designed.

// function allocates mutable set which doesn't retain references.NSMutableSet* AllocNotRetainedMutableSet() {    CFMutableSetRef setRef = NULL;    CFSetCallBacks notRetainedCallbacks = kCFTypeSetCallBacks;    notRetainedCallbacks.retain = NULL;    notRetainedCallbacks.release = NULL;    setRef = CFSetCreateMutable(kCFAllocatorDefault,    0,&notRetainedCallbacks);    return (__bridge NSMutableSet *)setRef;}// test object for debug deallocation@interface TestObj : NSObject@end@implementation TestObj- (id)init {   self = [super init];   NSLog(@"%@ constructed", self);   return self;}- (void)dealloc {   NSLog(@"%@ deallocated", self);}@end@interface MainViewController () {   NSMutableSet *weakedSet;   NSMutableSet *usualSet;}@end@implementation MainViewController- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];    if (self) {        // Custom initialization      weakedSet = AllocNotRetainedMutableSet();      usualSet = [NSMutableSet new];   }    return self;}- (IBAction)addObject:(id)sender {   TestObj *obj = [TestObj new];   [weakedSet addObject:obj]; // store unsafe unretained ref   [usualSet addObject:obj]; // store strong ref   NSLog(@"%@ addet to set", obj);   obj = nil;   if ([usualSet count] == 3) {      [usualSet removeAllObjects];  // deallocate all objects and get old fashioned crash, as it was required.      [weakedSet enumerateObjectsUsingBlock:^(TestObj *invalidObj, BOOL *stop) {         NSLog(@"%@ must crash here", invalidObj);      }];   }}@end

Output:

2013-06-30 00:59:10.266 not_retained_collection_test[28997:907] constructed 2013-06-30 00:59:10.267 not_retained_collection_test[28997:907] addet to set 2013-06-30 00:59:10.581 not_retained_collection_test[28997:907] constructed 2013-06-30 00:59:10.582 not_retained_collection_test[28997:907] addet to set 2013-06-30 00:59:10.881 not_retained_collection_test[28997:907] constructed 2013-06-30 00:59:10.882 not_retained_collection_test[28997:907] addet to set 2013-06-30 00:59:10.883 not_retained_collection_test[28997:907] deallocated 2013-06-30 00:59:10.883 not_retained_collection_test[28997:907] deallocated 2013-06-30 00:59:10.884 not_retained_collection_test[28997:907] deallocated 2013-06-30 00:59:10.885 not_retained_collection_test[28997:907] * -[TestObj respondsToSelector:]: message sent to deallocated instance 0x1f03c8c0

Checked with iOS versions 4.3, 5.1, 6.2. Hope it will be useful to somebody.