[swift-users] Using NSObject subclass instance as key in Dictionary

[Dmitri Gribenko]

On Thu, Dec 17, 2015 at 2:17 PM, Etan Kissling via swift-users <
swift-users at swift.org> wrote:

> I want to use instances of a custom class as Dictionary key.
> This requires the class to conform to Hashable.
>
> func ==(lhs: KeyType1, rhs: KeyType1) -> Bool { return lhs.id == rhs.id }
>
> final class KeyType1: Hashable, CustomStringConvertible {
>     let id: String
>     init(id: String) { self.id = id }
>     var hashValue: Int { return id.hashValue }
>     var description: String { return id }
> }
>
>
> Now I can use KeyType1 instances as key in Dictionary.
>
> var collection1 = [KeyType1(id: "foo") : NSObject()]
>
>
> Testing works fine:
>
>     let key = collection1.first!.0
>     print("        Key stored in collection: \(unsafeAddressOf(key)) -- \(
> key)")
>
>
>     let keyCopy = KeyType1(id: key.id)
>     print("                        Key copy: \(unsafeAddressOf(keyCopy))
> -- \(keyCopy)")
>
>
>     print("                      Keys equal: \(key == keyCopy)")
>     print("               Hash values equal: \(key.hashValue == keyCopy.
> hashValue)")
>     print("     Collection has item for key: \(collection1[key] != nil)")
>     print("Collection has item for key copy: \(collection1[keyCopy] != nil
> )")
>
> *        Key stored in collection: 0x0000608000043d80 -- foo*
> *                        Key copy: 0x00006080000440b0 -- foo*
> *                      Keys equal: true*
> *               Hash values equal: true*
> *     Collection has item for key: true*
> *Collection has item for key copy: true*
>
>
>
> Next, I repeat the same set up -- but this time KeyType is a descendant of
> NSObject.
>
> func ==(lhs: KeyType2, rhs: KeyType2) -> Bool { return lhs.id == rhs.id }
>
> final class KeyType2: NSObject { // NSObject conforms to Hashable and
> CustomStringConvertible.
>     let id: String
>     init(id: String) { self.id = id; super.init() }
>     override var hashValue: Int { return id.hashValue }
>     override var description: String { return id }
> }
>
> Again, I create a Dictionary based on this key class.
>
> var collection2 = [KeyType2(id: "foo") : NSObject()]
>
> Using the same tests, they fail now.
>
>     let key = collection2.first!.0
>     print("        Key stored in collection: \(unsafeAddressOf(key)) -- \(
> key)")
>
>     let keyCopy = KeyType2(id: key.id)
>     print("                        Key copy: \(unsafeAddressOf(keyCopy))
> -- \(keyCopy)")
>
>     print("                      Keys equal: \(key == keyCopy)")
>     print("               Hash values equal: \(key.hashValue == keyCopy.
> hashValue)")
>     print("     Collection has item for key: \(collection2[key] != nil)")
>     print("Collection has item for key copy: \(collection2[keyCopy] != nil
> )")
>
> *        Key stored in collection: 0x0000608000044080 -- foo*
> *                        Key copy: 0x00006080000440e0 -- foo*
> *                      Keys equal: true*
> *               Hash values equal: true*
> *     Collection has item for key: true*
> *Collection has item for key copy: false*
>
>
> What am I missing here?
>

The == overload in the second case is not the one that gets put into the
protocol witness table.  When you call == on two instances of your type,
upcast to NSObject, the isEqual() method is called.

We are aware of this issue and it will be fixed when we move operators into
types.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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