Question

In your opinion if I have a singleton subclass of NSObject being initialised with parameters like this:

- (MyObject *) initWithSomeParam:(NSString *)param{
    self = [super init];
    if (SharedInstance == nil){
        SharedInstance = [super init];
        SharedInstance.someProperty = param;
    }
    return self;
}

+ (MyObject *) objectWithSomeParam:(NSString *)param{
    return [[self alloc] initWithSomeParam:param];
    // Will the alloc cause a leak?
}

The user doesn’t have access to the instance method, just the class. Thanks.

Answer 1

That’s not the normal way of implementing a singleton and you are breaking the convention of init. Better would be to create a sharedInstance class method and leave the initWithParam method to be more conventional:

static MyObject *_sharedInstance = nil;

+ (MyObject *)sharedInstance:(NSString *)param
{
    if (_sharedInstance == nil)
    {
        _sharedInstance = [MyObject alloc] initWithParam:param];
    }
    return _sharedInstance;
}

// This must be called during app termination to avoid memory leak
+ (void)cleanup
{
    [_sharedInstance release];
    _sharedInstance = nil;
}

- (id)initWithParam:(NSString *)param
{
    self = [super init];
    if (self != nil)
    {
        self.someProperty = param;
    }
    return self;
}

However, even that doesn’t seem very comfortable; i.e. what happens if the user calls sharedInstance with a different parameter? Perhaps you want to keep a NSMutableDictionary of the initialized objects and create/return them depending on the parameter?

If so, you would do:

static NSMutableDictionary _sharedInstances = [[NSMutableDictionary alloc] init];

+ (MyObject *)sharedInstance:(NSString *)param
{
    MyObject *obj = [_sharedInstances objectForKey:param];
    if (obj == nil)
    {
        obj = [[MyObject alloc] initWithParam:param];
        [_sharedInstances setObject:obj forKey:param];
    }
    return obj;
}

// This must be called during app termination to avoid memory leak
+ (void)cleanup
{
    [_sharedInstances release];
    _sharedInstances = nil;
}