> 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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