Question

If I want to return an immutable array like this + (NSArray *)ids but inside this method I’m declaring a NSMutableArray because I want to sort it using -sortUsingSelector:.

Returning this method works perfect.

• But is it “okay” to write code that declares that the return method should be one type and the actually type is another?

• Does this work because NSMutableArray is a subclass of NSArray?

• Is the actual return value an NSArray or an NSMutableArray?

Answer 1

(…) is it “okay” to write code that declares that the return method should be one type and the actualy type is another?

Yes, it is provided the types are compatible; see the next paragraph. In fact, there are some cases in Cocoa (notably class clusters) that do that as well.

Does this work because NSMutableArray is a subclass of NSArray?

Exactly. Since NSMutableArray is a subclass of NSArray, it inherits all its methods and declared properties, so it publicly behaves like NSArray. This is called the Liskov substitution principle.

Is the actual return value a NSArray or a NSMutableArray?

The return value is whatever you’re returning. Since you’re returning an NSMutableArray, it’s an NSMutableArray. From the caller perspective, it is an object that can be used like an NSArray.