Missing sentinel in method dispatch

Question

I want to create a subclass of NSMutableArray and need to override the -initWithObjects: method.

But How to call [super xxx];?

- (id) initWithObjects:(id)firstObj, ... {
    [super initWithObjects:firstObj]; // Error: Missing sentinel in method dispatch
    // Error: The result of a delegate init call must be immediately returned or assigned to "self"

}

Thanks.

Answer 1

Then "missing sentinel" message refers to the missing nil termination. In fact, according to font-of-all-knowledge-Wikipedia:

The name of the nil that terminates a variable length list of parameters in Objective-C

also: Sentinel node, an object to represent the end of a data structure also: Sentinel value, a value used to terminate a loop also: In network protocols such as Bisync, sentinel values indicate where frames start and end

Answer 2

You can't. As discussed in the documentation for NSArray:

You might want to implement an initializer for your subclass that is suited to the backing store that the subclass is managing. The NSArray class does not have a designated initializer, so your initializer need only invoke the init method of super. The NSArray class adopts the NSCopying, NSMutableCopying, and NSCoding protocols; if you want instances of your own custom subclass created from copying or coding, override the methods in these protocols.

So you can assign self = [super init]; and add the objects from your initialiser to the resulting object. Indeed, because of the way that NSArray is implemented, calling any -initWith… method is likely to return an instance of a different NSArray subclass.

Notice that the documentation also discusses alternatives to subclassing NSArray that may be easier, more reliable or better in some other way.

Answer 3

Subclassing NSArray/NSMutableArray doesn't work like subclassing most classes. NSArray is a class cluster, please see subclassing notes from the NSArray documentation.

Now, for your specific question, subclassing va_list methods is a bit tricky, there are a number of ways to handle this. The 'safest' would be to process your va_list into an NSArray and pass that into another method that dealt with whatever you wanted. The other, slightly less portable, slightly hackier way is to create a new va_list list on the stack to pass through.

    id __unsafe_unretained * stack = (typeof(stack))calloc(numOfObjects, sizeof(id));
    //filloutStack
    [super initWithObjects:*stack, nil];
    free(stack);

Answer 4

Subclassing Apples Collection classes isn't that difficult — if you use a tiny trick (see also: cocoawithlove).

A subclass is a "is-a" relationship in object-orientated Design. But there are also "has-a" relationships, i.e. wrappers.

If you would try to create a subclass of NSArray by using a pure is-a relationship, I guess, it would be kind of hard, as you would have to do C-level memory management.

But if you add a has-a relationship — or: create a wrapper — at the same time, you can the subcalssing quite easily: Just make your custom array class have a member of a regular NSArray. Now override its method by forwarding the calls to the member object. I showed this in this post, where I just add objects, that pass a certain test.

But you will see, that I didn't implement the method you talked about correctly, but I raise a error. The reason is: that method is a variadic methods, that has a variable number of objects you can pass in — and to handle this, you have to to a bit of work. cocoawithlove has an great article about it.

For you — if using that has-a trick — it could look like

- (id) initWithObjects:(id)firstObj, ... {

    if (self = [super init]) {
        _realArray = [[NSMutableArray alloc] initWithCapacity:1];
    }

    va_list args;
    va_start(args, firstObj);
    for (id obj = firstObj; obj != nil; obj = va_arg(args, id))
    {
        [self.realArray addObject:obj];
    }
    va_end(args);
    return self;
}

Answer 5

Try

self = [super initWithObjects:firstObj,nil];