Setting static inside its method call (with singleton pattern)

Question

is smth. like this legit? it compiles and looks running ok, but is it ok? (aim setting myself to nil, inside my method)

i mean iam setting myself static to nil, in a method

static MyClass * StaticInstance = nil;

+ (MyClass *) sharedStaticInstance 
{
    if (StaticInstance == nil) {
        StaticInstance = [[MyClass alloc] init];
    }

    return StaticInstance;
}

- (void) killStaticSelf
{
    StaticInstance = nil;
}

and later

[[MyClass sharedStaticInstance] doSmth]; // our static instance is created
[[MyClass sharedStaticInstance] killStaticSelf]; // now its killed inside itself method
[[MyClass sharedStaticInstance] doSmth]; // now it should recreate again

Apurv · Accepted Answer · 2012-02-09 11:19:28Z

2

Its having a memory leak. You should dealloc the StaticInstance first and then you should assign nil to it.

answered Feb 9, 2012 at 11:19

Apurv

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Comments

Aram Kocharyan · Accepted Answer · 2012-02-09 11:20:12Z

1

Yes, that's how it's done. I use the sharedStaticInstance often, though I don't usually create a destructor it's probably a good idea, as long as all references to the shared instance in this class pass through sharedStaticInstance first.

EDIT: I just noticed that killStaticSelf is an instance method - it should be a class method I believe, but there shouldn't be any issue either way.

[MyClass killStaticSelf];

Even as the function stack closes, since sending messages to nil doesn't cause issues in Objective-C.

answered Feb 9, 2012 at 11:20

Aram Kocharyan

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AndersK · Accepted Answer · 2012-02-09 11:35:59Z

1

your sharedInstance method is not thread safe so you could get a race condition in this code:

if (StaticInstance == nil) {
    StaticInstance = [[MyClass alloc] init];
}

- (void) killStaticSelf
{
    StaticInstance = nil;
}

the above code has a leak since you do not provide StaticInstance as a retain property (apparently). You could instead wrap your singleton code in a property but that uses the same static instance.

edited Feb 9, 2012 at 11:35

answered Feb 9, 2012 at 11:24

AndersK

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Comments

Eugene · Accepted Answer · 2012-02-09 11:17:39Z

0

It's legit, but you need to release the variable before setting it to nil, to avoid memory leaks if you're not using ARC.

Though comprehension of such singleton usage logic is beyond my humble brain abilities.

answered Feb 9, 2012 at 11:17

Eugene

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