IB compliant Singleton in ARC

Singleton.h

@interface Singleton : NSObject

+ (instancetype)sharedInstance;

@end

Singleton.m

@implementation Singleton

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

+ (instancetype)allocWithZone:(NSZone *)zone {
    return [self sharedInstance];
}

+ (instancetype)alloc {
    return [self sharedInstance];
}

- (instancetype)init {
    return self;
}

+ (instancetype)_alloc {
    return [super allocWithZone:NULL];
}

- (instancetype)_init {
    return [super init];
}

@end