Copyable1.swift

protocol Copyable {
    func copy() -> Self
}

class Class1: Copyable {
    var variable1: String
    
    init(variable1: String) {
        self.variable1 = variable1
    }
    
    func copy() -> Self {
        return Class1(variable1: self.variable1)
    }
}

class Class2: Class1 {
    var variable2: String
    
    init(variable1: String, variable2: String) {
        self.variable2 = variable2
        super.init(variable1: variable1)
    }
    
    override func copy() -> Self {
        return Class2(variable1: self.variable1, variable2: self.variable2)
    }
}

let var1: Class1 = Class2(variable1: "Bla", variable2: "Blub")

var1.copy()

Copyable2.swift

protocol Copyable {
	init(_ instance: Self)
}

extension Copyable {
	final func copy() -> Self {
		return Self.init(self)
	}
}

class Class1: Copyable {
    var variable1: String
    
    init(variable1: String) {
        self.variable1 = variable1
    }
    
    required init(_ instance: Class1) {
        self.variable1 = instance.variable1
    }
}

class Class2: Class1 {
    var variable2: String
    
    init(variable1: String, variable2: String) {
        self.variable2 = variable2
        super.init(variable1: variable1)
    }
    
    required init(_ instance: Class2) {
        self.variable2 = instance.variable2
        super.init(instance)
    }
    
    required init(_ instance: Class1) {
        fatalError("Invalid instance")
    }
}

let var1: Class1 = Class2(variable1: "Bla", variable2: "Blub")

var1.copy()

Copyable3.swift

protocol Copyable {
    func copy() -> Self
}

func copycast<T>(obj: AnyObject) -> T {
    return obj as! T
}

class Class1: Copyable {
    var variable1: String
    
    init(variable1: String) {
        self.variable1 = variable1
    }
    
    func copy() -> Self {
        return copycast(Class1(variable1: self.variable1))
    }
}

class Class2: Class1 {
    var variable2: String
    
    init(variable1: String, variable2: String) {
        self.variable2 = variable2
        super.init(variable1: variable1)
    }

    override func copy() -> Self {
        return copycast(Class2(variable1: self.variable1, variable2: self.variable2))
    }
}

let var1: Class1 = Class2(variable1: "Bla", variable2: "Blub")

var1.copy()