[swift-users] Subtract a set of a subclass?

[Nick Brook]

Hi Zhao

Thanks for your response.

I understand your point, but when using third party classes I don’t know if the hash values are comparable, but for example I may want to have a set of ‘data' (NSData) and a subset of ‘mutable data' (NSMutableData), which point to the same objects. As a user of swift I would expect to be able to subtract Set<NSMutableData> from Set<NSData>.

Your last example perhaps works in Swift 3, so this may be fixed now, but in Swift 2 you get the error

Cannot invoke 'subtract' with an argument list of type '(Set<Bar>)’

Perhaps Swift 3 supports it with some additional safety around hashValue overriding or something.

Thanks

Nick

> On 1 Sep 2016, at 04:00, Zhao Xin <owenzx at gmail.com> wrote:
> 
> I don't see the point. For example if an element in Set<B> and another element in Set<A> are with a same hash value. Neither of the elements should be subtracted. As they are in different types. And hash values between different types are not guaranteed to be comparable. 
> 
> import Foundation
> 
> class Foo:Hashable {
>     var value: Int
>     
>     public var hashValue: Int {
>         return value
>     }
>     
>     public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>         return lhs.value == rhs.value
>     }
>     
>     required init(_ value:Int) {
>         self.value = value
>     }
> }
> 
> class Bar:Foo {
>     override public var hashValue: Int {
>         return value * 10
>     }
> }
> 
> let foo = Foo(10)
> let bar = Bar(10)
> 
> print(foo.hashValue) // 10
> print(bar.hashValue) // 100
> print((bar as Foo).hashValue) // 100 instead of 10
> 
> print(foo == bar) // true
> print(foo.hashValue == bar.hashValue) // false
> 
> As you can see in the above code, although `foo == bar` is true, `foo.hashValue == bar.hashValue` is not guaranteed to be true. As far as I know, Set<T> uses hash values instead of equations to compare the elements. So the results of a super class and its sub class are not guaranteed. Also, as `(bar as Foo).hashValue` is always the result of its own class, you can't get the results you want through casting.
> 
> 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)
> fooSet.forEach { print("\(type(of:$0)), value:\($0.value)") }
> /*
>  Foo, value:10
>  Foo, value:9
>  Foo, value:8 // Here is a mystery, Foo(7) is unreasonably missing.
> */
> 
> However, if you can guarantee the hash values are comparable, you still can get the results you want.
> 
> class Foo:Hashable {
>     var value: Int
>     
>     public var hashValue: Int {
>         return value
>     }
>     
>     public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>         return lhs.value == rhs.value
>     }
>     
>     required init(_ value:Int) {
>         self.value = value
>     }
> }
> 
> class Bar:Foo {
>     var name = "bar"
> }
> 
> 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)
> fooSet.forEach { print("\(type(of:$0)), value:\($0.value)") }
> /*
>  Foo, value:10
>  Foo, value:9
> */
> 
> 
> Zhaoxin
> 
> On Thu, Sep 1, 2016 at 8:31 AM, Nick Brook via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> I have a set, Set<A> and a subset of that, Set<B>, where B: A. I want to subtract Set<B> from Set<A>, but the subtract function signature specifies that the set elements must be the same type (S.Generator.Element == Element). I guess this is because Element is not required to be a class, simply hashable, therefore inheritance is not guaranteed? Is there any way this could be implemented in Set, in an extension, or what would be the most efficient way to perform that operation?
> 
> Thanks
> 
> Nick
> 
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> 

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