singleton using init override

singleton

@interface blah : blah
-(id)init NS_UNAVAILABLE;
@end 

@implementation
-(id)init{abort();}

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

+(instancetype)sharedInstance {
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedInstance = [[self alloc] internalInit];
    });
    return sharedInstance;
}
@end