[swift-users] Set with element of NSObject subclasses didn't work as expected

[Saagar Jha]

NSObject’s hash (which Set uses) is done via ObjectIdentifier, which IIRC uses something along the lines of the object's address. That’s why you’re getting the behavior you’re seeing.

Saagar Jha

> On Mar 28, 2017, at 20:20, Zhao Xin via swift-users <swift-users at swift.org> wrote:
> 
> Turns out that for `NSObject`, protocol `Equatable` wasn't used. Instead, it used `NSObjectProtocol.isEqual(_ object: Any?)`. Also, override `func isEqual(_ object: Any?) -> Bool` requires to override `var hash: Int { get }` as well.
> 
> I think this behavior should be mentioned in Swift docs or manual in `Set` section.
> 
> Below code works. 
> 
> class Bar:NSObject {
>     let value:Int
>     
>     override public var hashValue: Int { return value }
>     public static func ==(lhs: Bar, rhs: Bar) -> Bool {
>         return lhs.value == rhs.value
>     }
>     // required by NSObjectProtocol
>     override func isEqual(_ object: Any?) -> Bool {
>         if let rhs = object as? Bar {
>             return self == rhs
>         }
>         return false
>     }
>     override var hash: Int { return self.hashValue }
>     
>     init(_ value:Int) {
>         self.value = value
>     }
> }
> 
> let barSetA:Set = [Bar(8), Bar(9)]
> let barSetB:Set = [Bar(9), Bar(10)]
> let barResultC = barSetA.intersection(barSetB) // {{NSObject, value 9}}
> let barResultD = barSetA.subtracting(barSetB) // {{NSObject, value 8}}
> 
> Gladly I find it here <http://stackoverflow.com/questions/32726524/swift-2-0-set-not-working-as-expected-when-containing-nsobject-subclass>. 
> 
> Zhaoxin
> 
> 
> 
> On Wed, Mar 29, 2017 at 3:50 AM, Zhao Xin <owenzx at gmail.com <mailto:owenzx at gmail.com>> wrote:
> Please see the code first.
> 
> import Foundation
> 
> class Foo:Hashable {
>     let value:Int
>     
>     public var hashValue: Int { return value }
>     public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>         return lhs.value == rhs.value
>     }
>     
>     init(_ value:Int) {
>         self.value = value
>     }
> }
> 
> let fooSetA:Set = [Foo(8), Foo(9)]
> let fooSetB:Set = [Foo(9), Foo(10)]
> let fooResultC = fooSetA.intersection(fooSetB) // {{value 9}}
> let fooResultD = fooSetA.subtracting(fooSetB) // {{value 8}}
> 
> 
> class Bar:NSObject {
>     let value:Int
>     
>     override public var hashValue: Int { return value }
>     public static func ==(lhs: Bar, rhs: Bar) -> Bool {
>         return lhs.value == rhs.value
>     }
>     
>     init(_ value:Int) {
>         self.value = value
>     }
> }
> 
> let barSetA:Set = [Bar(8), Bar(9)]
> let barSetB:Set = [Bar(9), Bar(10)]
> let barResultC = barSetA.intersection(barSetB) // Set([])
> let barResultD = barSetA.subtracting(barSetB) // {{NSObject, value 9}, {NSObject, value 8}}
> 
> 
> Behaviors of `func intersection(Set<Set.Element>)` and `func subtracting(Set<Set.Element>)` were different between normal Swift class and NSObject subclasses. I had thought they should be the same. It seemed that Set<NSObject> relied on addresses of NSObject instances instead of their hashValues. That made the Set useless.
> 
> Swift version: 3.1 (swiftlang-802.0.48 clang-802.0.48)
> Xcode 8.3 (8E162)
> 
> Zhaoxin
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20170328/95a9cdb9/attachment.html>