Array from set: why does NSSet use allObjects, while NSOrderedSet uses array?

Question

In Foundation, if I want to convert a set to an NSArray, I can use:

• -[NSSet allObjects]
• -[NSOrderedSet array]

Why are these different?

Answer 1

Speculation, but:

Because when NSSet was created the only other major collection type was NSArray, which was (and still is, largely) the most common collection type. So a method called "allObjects" would obviously return an NSArray.

When NSOrderedSet was added much more recently, it had to deal with the existence of prior collections - primarily, NSArray and NSSet. So an "allObjects" method would be ambiguous. Ergo it has two methods, -array and -set.

And/or, the -array and -set methods return proxies to what are likely the same or similar classes used internally. So in a functional sense they're a little different - those proxies will see mutations made on the original NSOrderedSet. -allObjects on the other hand does not - it genuinely creates an independent array, since its internal storage is likely a hashtable or similar that isn't readily proxied.

Answer 2

While there are other differences†, .allObjects does not imply a definite ordering, and .array does; and that's exactly what you are getting.

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† .array returns a live proxy of the underlying NSOrderedSet, and if the underlying ordered set changes, the proxy will change with it.