Hi Jordan, Thanks for the advice.
What if a subclass does implement hashValue differently? It seems you are saying a subclass should never override hashValue? Should Set not compare elements with == instead of hashValue? Thanks Nick M: +44 (0)7986 048 141 W: http://nickbrook.me <http://nickbrook.me/> > On 1 Sep 2016, at 23:55, Jordan Rose <jordan_r...@apple.com> wrote: > >> >> On Sep 1, 2016, at 15:44, Zhao Xin via swift-users <swift-users@swift.org >> <mailto:swift-users@swift.org>> wrote: >> >> Hi Nick, >> >> Glad to help. >> >> but when using third party classes I don’t know if the hash values are >> comparable >> >> You can create an extension with a convenient init(:), which creates a new >> instance of the super class basing on the instance of the sub class. That >> will guarantee the subtraction. Below code works in Xcode 7.3.1 with Swift >> 2.2. >> >> import Foundation >> >> func ==(lhs: Foo, rhs: Foo) -> Bool { >> return lhs.id == rhs.id >> } >> >> class Foo:Hashable { >> let id:Int >> var hashValue: Int { >> return id >> } >> >> required init(_ id:Int) { >> self.id = id >> } >> } >> >> class Bar:Foo { >> override var hashValue: Int { >> return id * 5 >> } >> } >> >> var fooSet:Set<Foo> = [Foo(10), Foo(9), Foo(8), Foo(7)] >> var barSet:Set<Bar> = [Bar(8), Bar(7), Bar(6), Bar(5)] >> >> //fooSet.subtract(barSet) // error: cannot invoke 'subtract' with an >> argument list of type '(Set<Bar>)' >> fooSet = fooSet.subtract(barSet as Set<Foo>) // works, but not what we want >> fooSet.forEach { print("\($0.dynamicType), id:\($0.id)") } >> /* >> Foo, id:7 >> Foo, id:10 >> Foo, id:9 >> */ > > This isn't really a sensible thing to do. The rules for Hashable require that > `a == b` implies `a.hashValue == b.hashValue`, and `a.hashValue != > b.hashValue` implies `a != b`. If you break these rules you're going to have > problems no matter what static types you're using. > > Upcasting from Set<Bar> to Set<Foo> is the most concise way to solve this > problem. > > Jordan
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