I believe if B inherits A, they are not the same type. So the rule doesn't apply here.
Zhaoxin On Fri, Sep 2, 2016 at 7:02 AM, Nick Brook <nrbr...@gmail.com> wrote: > 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 > > 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> > 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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