singleton.m

static BOOL isLoaded;

/**
   Singleton, shared instance
 **/
+ (id)sharedInstance
{
    __strong static id _sharedObject = nil;
	
	if(isLoaded == NO) {
		isLoaded = YES;
		_sharedObject = [[self alloc] init];
	}

    return _sharedObject;
}

singleton2.m

// Probably a better way.

/**
   Singleton, shared instance
 **/
+ (id)sharedInstance
{
    static dispatch_once_t p = 0;
    __strong static id _sharedObject = nil;
    dispatch_once(&p, ^{
        _sharedObject = [[self alloc] init];
    });
    return _sharedObject;
}