CODEWITHSUNDEEP
swiftprotocolsextension-methodsswift4swift-extensions
Mark A. Donohoe
Mark A. Donohoe

Reputation: 30408

In Swift 4, can you write an extension that applies only to things that adhere to multiple protocols?

Consider these protocols

protocol NamedThing{
    var name:String{ get }
}

protocol ValuedThing{

    associatedtype ValueType

    var value:ValueType{ get }
}

And these structs...

struct TestThingA : NamedThing {
    let name = "TestThing"
}

struct TestThingB : ValuedThing {

    typealias ValueType = Int

    let value = 4
}

struct TestThingC : NamedThing, ValuedThing {

    typealias ValueType = Int

    let name = "TestThing"
    let value = 4
}

I'm trying to write an extension that would only apply to struct TestThingC because it adheres to both protocols.

None of these work, of course...

extension NamedThing & ValuedThing{
    func test(){
        print("Named thing \(name) has a value of \(value)")
    }
}

extension Any where NamedThing & ValuedThing {
    func test(){
        print("Named thing \(name) has a value of \(value)")
    }
}

extension Any where Self is NamedThing, Self is ValuedThing{

    func test(){
        print("Named thing \(name) has a value of \(value)")
    }
}

extension Any where Self == NamedThing, Self == ValuedThing{

    func test(){
        print("Named thing \(name) has a value of \(value)")
    }
}

So how does one write an extension that applies to items which adhere to both (multiple) protocols?

Upvotes: 3

Views: 108

Answers (2)

Yannick
Yannick

Reputation: 3278

I have always solved these issues by creating a new protocol that inherits from the other protocols.

protocol NamedValueThing: NamedThing, ValuedThing { }
extension TestThingC: NamedValueThing { }

extension NamedValueThing {
    func test() {
        print("\(value) \(name)")
    }
}

//TestThingA().test() //error: value of type 'TestThingA' has no member 'test'
//TestThingB().test() //error: value of type 'TestThingB' has no member 'test'
TestThingC().test() //4 TestThing

Upvotes: 1

Martin R
Martin R

Reputation: 539795

You could define a restricted extension to one of the protocols:

extension NamedThing where Self: ValuedThing {
    func test(){
        print("Named thing \(name) has a value of \(value)")
    }
}

TestThingC().test() // compiles

TestThingA().test() // error: Type 'TestThingA' does not conform to protocol 'ValuedThing'
TestThingB().test() // error: Value of type 'TestThingB' has no member 'test'

Upvotes: 4

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