Correct Singleton Pattern Objective C (iOS)?

Question

I found some information in the net to create a singleton class using GCD. Thats cool because it's thread-safe with very low overhead. Sadly I could not find complete solutions but only snippets of the sharedInstance method. So I made my own class using the trial and error method - and et voila - the following came out:

@implementation MySingleton  // MARK: - // MARK: Singleton Pattern using GCD  + (id)allocWithZone:(NSZone *)zone { return [[self sharedInstance] retain]; } - (id)copyWithZone:(NSZone *)zone { return self; } - (id)autorelease { return self; } - (oneway void)release { /* Singletons can't be released */ } - (void)dealloc { [super dealloc]; /* should never be called */ } - (id)retain { return self; } - (NSUInteger)retainCount { return NSUIntegerMax; /* That's soooo non-zero */ }  + (MySingleton *)sharedInstance {     static MySingleton * instance = nil;      static dispatch_once_t predicate;        dispatch_once(&predicate, ^{         // --- call to super avoids a deadlock with the above allocWithZone         instance = [[super allocWithZone:nil] init];     });      return instance; }  // MARK: - // MARK: Initialization  - (id)init {     self = [super init];     if (self)      {         // Initialization code here.     }     return self; }  @end 

Please feel free to comment and tell me if I've missing something or doing something completely wrong ;)

Cheers Stefan

Answer 1

Keep it simple:

+(instancetype)sharedInstance {     static dispatch_once_t pred;     static id sharedInstance = nil;     dispatch_once(&pred, ^{         sharedInstance = [[self alloc] init];     });     return sharedInstance; }  - (void)dealloc {     // implement -dealloc & remove abort() when refactoring for     // non-singleton use.     abort(); } 

That is it. Overriding retain, release, retainCount and the rest is just hiding bugs and adding a bunch of lines of unnecessary code. Every line of code is a bug waiting to happen. In reality, if you are causing dealloc to be called on your shared instance, you have a very serious bug in your app. That bug should be fixed, not hidden.

This approach also lends itself to refactoring to support non-singleton usage modes. Pretty much every singleton that survives beyond a few releases will eventually be refactored into a non-singleton form. Some (like NSFileManager) continue to support a singleton mode while also supporting arbitrary instantiation.

Note that the above also "just works" in ARC.