Question

I have the following:

static char associated_obj_key;
id anObj;

NSMutableArray *a = [[NSMutableArray alloc] init];
objc_setAssociatedObject(anObj, &associated_obj_key, a, OBJC_ASSOCIATION_RETAIN);
[a release];

And I’d like to know: how to add objects to the NSMutableArray inside an associative reference for the anObj object? Is it such a thing even possible?

Can I even do something like [objc_getAssociatedObject(anObj, &associated_obj_key) addObject:something];?

Answer 1

Yes, but you would have to have the same key all over the places. As using an address of some static variable is indeed a recommended way, I suggest factoring out the single method for obtaining a reference of an associated object (creating if necessary). I use the following:

- (NSMutableArray*) getOrCreateArray 
{ 
    static const char key;
    NSMutableArray* arr = objc_getAssociatedObject(self, &key); 
    if (!arr) 
    { 
        arr = [NSMutableArray array];
        objc_setAssociatedObject(self, &key, arr, OBJC_ASSOCIATION_RETAIN);
    }
    return arr;
}

- (void) someMethod
{
    id something = // ....
    [[self getOrCreateArray] addObject: something];
}