Implement alloc on a singleton
ios, objective-c
Solution
As @H2CO3 mentioned, your method of going about producing singletons is acceptable, however not threadsafe. The more traditional approach is to wrap your assignment and comparison in an `@synchronized` block so multiple thread access is reduced, however overriding `+alloc` is not the best way of going about implementing an already shaky pattern.
Problem
I'd like to have a singleton in my system but rather than have callers access it via some kind of 'sharedInstance' method, I'd like them to be able to be unaware that they are using a singleton, in other words, I'd like the callers to be able to say: ``` MyClass *dontKnowItsASingleton = [[MyClass alloc] init]; ``` To accomplish this, I've tried overriding alloc as follows: ``` // MyClass.m static MyClass *_sharedInstance; + (id)alloc { if (!_sharedInstance) { _sharedInstance = [super alloc]; } return _sharedInstance; } ``` My question is: is this okay? It seems to work, but I've never overridden alloc before. Also, if it's okay, could I always use this technique, rather than dispatch_once approach I have been doing? ... ``` + (id)sharedInstance { static SnappyTV *_sharedInstance = nil; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ _sharedInstance = [[self alloc] init]; }); return _sharedInstance; } ```