Static variable scope within class?

Question

Can anyone tell me what the scope of the static variable in the class below is?

@implementation SharedManager

static id myInstance = nil;

+(id)sharedInstance {
    if(myInstance == nil) {
        myInstance = [[self alloc] init];
    }
    return myInstance;
}

In a test I created an instance from the class and then released it, but noticed that on creating a second instance that the static was not nil (i.e. pointing to the previously released object) For the test I fixed this by overriding -(void)dealloc for the class.

-(void)dealloc {
    NSLog(@”_deal: %@”, [self class]);
    [super release]
    myInstance = nil
}

gary

David Gelhar · Accepted Answer · 2010-03-10 16:55:20Z

1

The scope of the variable is limited to the "SharedManager" class itself (since it's declared in the @implementation section, it will not be visible to subclasses).

The duration of the variable is "static" meaning that there's one copy of the variable associated with the class itself; it does not get created/destroyed when you alloc/dealloc instances of the class.

Also; if your class is intended to be thread-safe, you should do

@synchronized(self) {
if (myInstance == nil) {
    myInstance = [[self alloc] init];
}

to your sharedInstance method, to handle the case of two threads simultaneously calling sharedInstance.

answered Mar 10, 2010 at 16:55

David Gelhar

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2 Comments

fuzzygoat Over a year ago

Thank you David, yes I appreciate that, but in this case the singleton is simply for the model data, its a simple MVC iPhone application and I don't foresee and threading happening. I just wanted to make sure how it was working, hence the unneeded dealloc as I don't see it being released either

fuzzygoat Over a year ago

Just a quick note from my singleton travels, @synchronized([SharedManager class]) { might work better, or so I have been led to believe.

Seva Alekseyev · Accepted Answer · 2010-03-10 16:44:12Z

0

As far as I understand, the visibility scope of that variable is below its declaration within the current source file, and the lifetime is global. As if it's a C static variable.

In other news, you can write C functions within the @implementation block - they'll work like regular C functions.

There's no notion of "class static" variables in ObjC, AFAIK. This is not C++.

answered Mar 10, 2010 at 16:44

Seva Alekseyev

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2 Comments

fuzzygoat Over a year ago

Thank you, not sure about the "C Function" bit, the code shows "Objective-C Class Methods" ... but I could always be missing your point.

jlehr Over a year ago

Because Objective-C is C with O-O extensions, you can always do plain old C stuff, plus you can mix and match. What Seva is talking about is writing plain old C functions that contain Objective-C code. (Take a look at Apple's Foundation Functions reference for some examples.) As Seva points out, those can be placed anywhere in a .m file.

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