What are the memory implications of the sharedInstance pattern under ARC?

Question

I'm using the common singleton pattern as follows:

+ (SomeClass *)sharedInstance {
    static SomeClass *__sharedInstance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        __sharedInstance = [[SomeClass alloc] init];
    });

    return __sharedInstance;
}

I understand that the SomeClass object gets instantiated the first time I call [SomeClass sharedInstance], however, when does SomeClass get dealloc'd?

This is under ARC so when does the compiler release the SomeClass static object? Is it guaranteed to be around for the rest of the life of my app?

Answer 1

Since the declaration of __sharedInstance is inside sharedInstance 's scope, it cannot be accessed directly by any other methods. This means that no one else can set it to nil , and therefore it will be kept until your app will be terminated.

It's also worth mentioning that you cannot be certain that the dealloc method of your singleton will be called when the system frees your memory on termination, mainly because:

1. The OS itself may handle the termination of the process, without the Objective-C runtime calling your dealloc method before that happens.
2. Say that you do get a dealloc method, then you cannot predict the order of object destruction while terminating (such as frameworks that you use), making your code unreliable.