共有インスタンス生成

iPhone Exampleの TheElementsより

// we use the singleton approach, one collection for the entire application
static PeriodicElements *sharedPeriodicElementsInstance = nil;

+ (PeriodicElements*)sharedPeriodicElements {
    @synchronized(self) {
        if (sharedPeriodicElementsInstance == nil) {
            [[self alloc] init]; // assignment not done here
        }
    }
    return sharedPeriodicElementsInstance;
}
 
+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedPeriodicElementsInstance == nil) {
            sharedPeriodicElementsInstance = [super allocWithZone:zone];
            return sharedPeriodicElementsInstance;  // assignment and return on first allocation
        }
    }
    return nil; //on subsequent allocation attempts return nil
}
- (id)copyWithZone:(NSZone *)zone {
    return self;
}

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX;  //denotes an object that cannot be released
}

- (void)release {
    //do nothing
}

- (id)autorelease {
    return self;
}


// setup the data collection
- init {
	if (self = [super init]) {
		[self setupElementsArray];
	}
	return self;
}