Difference between (Type) and Type

Question

Currently, I'm writing a class and protocol named Control and Controllable .

Control must conform to Controllable .

Control will have an array of other Controllable s as a stack. Every time before adding Controllable to the stack, I should check whether the stack contains that element. If yes, move the element to the top of stack.

Okay. VERSION_1:

import Foundation

protocol Controllable: Equatable {
}

func ==<T: Controllable>(lhs: T, rhs: T) -> Bool {
    return ObjectIdentifier(lhs).uintValue == ObjectIdentifier(rhs).uintValue
}


class Control: Controllable {
    static var mainControl = Control()

    private init() {

    }

    private var stack: [Controllable] = []

    func addToStack(controllable: Controllable) {

    }
}

There is an error (Xcode 7.3):

return ObjectIdentifier(lhs).uintValue == ObjectIdentifier(rhs).uintValue

Cannot invoke initializer for type 'ObjectIdentifier' with an argument list of type '(T)'

QUESTION 1: Why? How can I construct ObjectIdentifier from lhs and rhs ?

---

VERSION_2:

import Foundation

protocol Controllable {
}

class Control: Controllable {
    static var mainControl = Control()

    private init() {

    }

    private var stack: [Controllable] = []

    func addToStack(controllable: Controllable) {
        stack.contains({ element in
            return ObjectIdentifier(element).uintValue == ObjectIdentifier(controllable).uintValue
        })
    }
}

There is an error (, again):

return ObjectIdentifier(element).uintValue == ObjectIdentifier(controllable).uintValue

Cannot invoke initializer for type 'ObjectIdentifier' with an argument list of type '((Controllable))'

QUESTION 2: Is (Controllable) tuple? How should I extract Controllable from the tuple?

---

_{Sorry for my English}

Answer 1

ObjectIdentifier only works on objects and metatypes as argument in the constructor.

Types that conforms to Controllable are not conforming to the type constraint in the ObjectIdentifier constructor.

init(_ x: AnyObject)
init(_ x: Any.Type)

see here .

You want to give the ObjectIdentifier the metatype of a type, like String.self. Or you want to give the ObjectIdentifier an object, like "hello". You could add the object constraint to the protocol like this:

import Foundation

protocol Controllable: Equatable, AnyObject {
}

func ==<T: Controllable>(lhs: T, rhs: T) -> Bool {
    return ObjectIdentifier(lhs).uintValue == ObjectIdentifier(rhs).uintValue
}