CODEWITHSUNDEEP
objective-cdesign-patternssingleton
Michal Shatz
Michal Shatz

Reputation: 1416

Objective C Singleton - why the initial nil assignment?

I am writing a singleton in ObjectiveC and saw this implementation in many sites I looked at. I really do not understand the nil assignment in the first raw. Since the block in dispatch_once (to my humble understanding...), how does this method return the previously assigned value (inside the block) and not nil.

 + (id)sharedManager {
        static MyManager *sharedMyManager = nil;
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            sharedMyManager = [[self alloc] init];
        });
        return sharedMyManager;
    }

P.S - I noticed this question was asked once here but was not resolved and non of the unswears explain this clearly - Why is assigned a nil to singleton's static variable

Upvotes: 0

Views: 225

Answers (3)

Martin R
Martin R

Reputation: 539745

The sharedMyManager = nil assignment is not necessary because static variables are automatically initialized to zero. You can simply remove it.

In addition, the dispatch_once() guarantees that sharedMyManager is assigned a value before it is used.

Also note that dispatch_once() uses the fact that the onceToken is initialized to zero automatically.

Upvotes: 5

rmaddy
rmaddy

Reputation: 318804

In C based languages, such as Objective-C, static variables are only initialized once. So the nil assignment only happens the very first time the code is executed.

Then the call to dispatch_once only happens once (that is its purpose), so the static variable is then set to the desired value just the one time.

Upvotes: 3

Merlevede
Merlevede

Reputation: 8170

If you don't initialize your static variable, it can initially have garbage data.

Static variables are initialized once. So in the +sharedManager message, the line where the static variable is declared is only run once, and in subsequent calls that line is skipped.

That's why this is the 'standard way' of using static variables inside functions.

Upvotes: -2

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