How careful are you with your return types in Objective-C?

Question

Say you have a method that returns a newly generated NSArray instance that is built internally with an NSMutableArray. Do you always do something like this:

- (NSArray *)someArray {
    NSMutableArray *mutableArray = [[NSMutableArray new] autorelease];
    // do stuff...
    return [NSArray arrayWithArray:mutableArray];  // .. or [[mutableArray copy] autorelease]
}

Or do you just leave the mutable array object as-is and return it directly because NSMutableArray is a subclass of NSArray:

- (NSArray *)someArray {
    NSMutableArray *mutableArray = [[NSMutableArray new] autorelease];
    // do stuff...
    return mutableArray;
}

Personally, I often turn a mutable array into an NSArray when I return from methods like this just because I feel like it's "safer" or more "correct" somehow. Although to be honest, I've never had a problem returning a mutable array that was cast to an NSArray, so it's probably a non-issue in reality - but is there a best practice for situations like this?

Answer 1

It's very common to return an NSMutableArray cast as an NSArray. I think most programmers would realize that if they downcast an immutable object and mutate it, then they're going to introduce nasty bugs.

Also, if you have an NSMutableArray ivar someMutableArray, and you return [NSArray arrayWithArray:someMutableArray] in a KVC accessor method, it can mess up KVO. You'll start getting "object was deallocated with observers still attached" errors.

Answer 2

NSArray is in fact a class cluster, not a type, anyway. So anywhere you see an NSArray, chances are it's already one of several different types anyway. Therefore the 'convert to NSArray' is somewhat misleading; an NSMutableArray already conforms to the NSArray interface and that's what most will deal with.

CocoaObjects fundamentals

In any case, given that you're returning an array (and not keeping it afterwards, thanks to the autorelease) you probably don't need to worry whether the array is mutable or not.

However, if you were keeping the array, then you might want to do this, to prevent the clients from changing the contents.

Answer 3

I used to do the return [NSArray arrayWithArray:someMutableArray], but I was slowly convinced that it doesn't offer any real benefit. If a caller of your API is treating a returned object as a subclass of the declared class, they're doing it wrong.

[NB: See bbum's caveat below.]