Upgrade an existing singleton to ARC

Question

In Objective-C, how to write a singleton with ARC? In ARC, it is not allowed to overwrite the release, autorelease, retain, retainCount methods, how to avoid a object to be released? I know without ARC, a classic singleton would like below:

@interface SingletonObject
+ (SingletonObject*)sharedObject;
@end

SingletonObject *sharedObj;

@implementation SingletonObject
+ (id)allocWithZone:(NSZone *)zone
{
    if (sharedObj == nil) {
        //So the code [[SingletonObject alloc] init] is equal with [SingletonObject sharedObject]
        sharedObj = [super allocWithZone:zone];
    }
    return sharedObj; 
}


+ (void)initialize
{
    if (self == [SingletonObject class]) {
        sharedObj = [[SingletonObject alloc] init];
    }
}

+ (SingletonObject*)sharedObject
{
    return sharedObj;
}
- (id)retain
{
    return self;
}

- (unsigned)retainCount
{
    return UINT_MAX;  //denotes an object that cannot be released
}

- (oneway void)release
{
    //do nothing
}

- (id)autorelease
{
    return self;
}

- (id)init {
    self  = [super init];
    if (self) {
       //... 
    }
    return self;
}

@end

Is it safe to just remove the retain, retainCount, release, autorelease methods? Thanks!

If wevah's answer helped you you should mark it as correct answer so it may help others. Just FYI. — Adrian P

bbum bbum · Accepted Answer · 2012-12-07T16:14:53

You only need one method to implement a class that supports the singleton pattern:

+ (instancetype)sharedInstance
{
    static id _sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _sharedInstance = [[self alloc] init];
    });
    return _sharedInstance;
}

Copy/paste that into any class and it'll have a shared instance. Any code beyond that is just added complexity that really isn't necessary. I'll sometimes add:

- (void)dealloc
{
     *(char*)0x42 = 'b';
     // no super, ARC all the way
}

That'll cause a very specific crash if my shared instance is ever deallocated due to a bug. (Yes, hex 0x42 is not 42, but it leaves a nice 0x000000042 in a register in the crash log, making it immediately identifiable what happened.)

Upgrade an existing singleton to ARC

2 Answers