CODEWITHSUNDEEP
objective-csingletonobjective-c-blocksgrand-central-dispatch
Nicolas Miari
Nicolas Miari

Reputation: 16256

GCD-style Singleton throws compile error... sometimes?

I'm implementing the textbook-singleton in Objective-C:

+ (instancetype) sharedSingleton
{
    id sharedInstance = nil;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{            
        sharedInstance = [[self alloc] init];
    });

    return sharedInstance;
}

...as I have been doing for a while. When changing an old @synchronized-syntax singleton to this kind, in an old project (open in the latest version of Xcode), I get the error:

Variable is not assignable (missing __block type specifier)

...pointing at the allocation line. I have the exact same code in many parts of other code, built and run with the same environment, and never an issue... What's going on? Should I prepend the __block qualifier and be done with it, or is there more than meets the eye here?

The only thing I can think of is, this old project I'm modernising right now has NOT been transitioned to ARC... (yet)

Upvotes: 0

Views: 147

Answers (1)

David Rönnqvist
David Rönnqvist

Reputation: 56635

You are missing static on the sharedInstance variable. That line should be

static id sharedInstance = nil;

Colin Wheelas has a good and short article about how to create singeltons using dispatch_once.

Upvotes: 4

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