CODEWITHSUNDEEP
iosobjective-cgrand-central-dispatchshared
Sishu
Sishu

Reputation: 1558

Singleton class static variable set to nil each time

I am making a singleton class for my use . I have seen code for singleton class is like this:

//First Example

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


 //Second Example

static SingletonSample *sharedObject;

+ (SingletonSample*)sharedInstance {
    if (sharedObject == nil) {
        sharedObject = [[super allocWithZone:NULL] init];
    }
    return sharedObject;
}

The seconds seems fine and understandable. But i am confused in First example where sharedMyManager is set to nil each time and also there is a allocation of shared manager each time, my doubt is that how will first example return the same reference of the class(Singleton).

Thanks.

Upvotes: 1

Views: 1707

Answers (3)

Valentin Shamardin
Valentin Shamardin

Reputation: 3658

sharedMyManager is set to nil each time

static variables are set only once.

there is a allocation of shared manager each time

You use dispatch_once, so it's incorrect too.

Read about dispatch_once here. And using GCD is faster and thread safe.

Upvotes: 1

trojanfoe
trojanfoe

Reputation: 122401

As it's static it will initially be set to nil, but in subsequent calls the value will remain to whatever you have set it to in previous calls.

The first example is better as it is thread safe. The second example doesn't even initialise sharedObject which is dangerous (I don't believe there is a guarantee that global variables are initialised to zero).

Upvotes: 2

Apurv
Apurv

Reputation: 17186

First of all when static is declared with in function, it is declared only once. So, the line

static MyManager *sharedMyManager = nil;

Will be executed only once when the function gets called for first time.

In the next line when you use dispath_once, it will be executed for only once. So the creation of sharedMyManager will be done once only. So, thats a perfect way to create a single ton class.

Upvotes: 5

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