what is the difference between singleton and common object init by alloc method?

Question

i am testing UITableView : (under ARC) code as below,

- (void)viewDidLoad
{
    [super viewDidLoad];
    UnitTest *unitTest = [[UnitTest alloc] init];

    UITableView *tableView = [[UITableView alloc] initWithFrame:self.view.bounds style:UITableViewStylePlain];
    tableView.delegate = unitTest;
    tableView.dataSource = unitTest;

    [self.view addSubview:tableView];

}

i have implement tableView delegate in Object UnitTest. when i run my project, it fetch a error:

libobjc.A.dylib`objc_msgSend:
0x30f13f68:  teq.w  r0, #0
0x30f13f6c:  beq    0x30f13faa               ; objc_msgSend + 66
0x30f13f6e:  push.w {r3, r4}
0x30f13f72:  ldr    r4, [r0]                // Thread 1: BAD_EXE_ACCESS ....
0x30f13f74:  lsr.w  r9, r1, #2
0x30f13f78:  ldr    r3, [r4, #8]
0x30f13f7a:  add.w  r3, r3, #8
0x30f13f7e:  ldr    r12, [r3, #-8]

and there is nothing in console.

when i change the line

UnitTest *unitTest = [[UnitTest alloc] init];

to

UnitTest *unitTest = [UnitTest sharedUnitTest]; // i have implement this singleton

Everything works OK!! why

Answer 1

You're in ARC, and it means something here. Basically, your unitTest will get deallocated sooner or later as you don't keep any reference to it. But still, it's both the delegate and dataSource of the tableView. Thus, after some time (to simplify), the tableView will try to call its delegate, but the unitTest will have been deallocated.

Thus, the simplest solution here, is to create a property for the unitTest : @property (nonatomic, strong) UnitTest *unitTest; in your header, synthesize it in your .m

Then, just use:

self.unitTest = [[UnitTest alloc] init];

It should fix it all. You probably don't need a singleton for this.

PS: the reason why the singleton fixes this issue is that the singleton will "live forever" (as long as the app is launched). As it never gets deallocated, no issue when the table view tries to call its delegate/dataSource.