protocol.swift

protocol MyProtocol {
    func doThe(thing: String)
}

extension MyProtocol {
    func doThe(thing: String = "") {
        print("protocol")
    }
}

class MyImplementation: MyProtocol {
    func doThe(thing: String = "") {
        print("implementation")
    }
}

class MyThing {
    let myProtocol: MyProtocol
    init(myProtocol: MyProtocol) {
        self.myProtocol = myProtocol
    }

    func doTheThing() {
        myProtocol.doThe(thing: "") // prints "implementation"
        myProtocol.doThe() // prints "protocol"
    }
}

MyThing(myProtocol: MyImplementation()).doTheThing()