[swift-users] ⁨Is it possible to store a set of heterogeneous items with protocol?

Glen Huang heyhgl at gmail.com
Tue Jul 11 05:09:54 CDT 2017


I want to store some heterogeneous items all conform to a protocol inside a set, is it something possible to do in swift?

I tried this example:

protocol Named: Hashable {
   var name: String { get }

extension Named {
   var hashValue: Int {
       return name.hashValue

   static func ==(lhs: Self, rhs: Self) -> Bool {
       return lhs.name == rhs.name

struct Foo: Named {
   var name = "foo"

struct Bar: Named {
   var name = "bar"

var item = Set<Named>()

But it failed at `Set<Named>()` where it complained "Using 'Named' as a concrete type conforming to protocol 'Hashable' is not supported”.

After watching the WWDC session "Protocol-Oriented Programming in Swift” by Dave Abrahams, I try to use protocols whenever possible. But I can’t seem to overcome this barrier. Set.Element must confirm to Hashable, which inherits from Equatable, which has self requirement, which ultimately means that Set.Element all must be of the same type. So it seems it’s impossible to have heterogeneous items using protocol. Is that the case?

My use case is this:

I have an object that can contain two sets of other objects:

class Parent {
   var foos: Set<Foo>
   var bars: Set<Bar>

I want to define a computed property “all” that is the union of the two sets. Foo and Bar conform to the same protocol. I wonder what return type I should use for the union? Do I have to go back to OOP and define a super class for Foo and Bar?


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