You can only use the computed variable with both setter and getter to overriding the stored property of the superclass

Question

You can only use the computed variable with both setter and getter to overriding the stored property of the superclass. I got this conclusion by trying the following code. I'm not sure I'm 100% correct. Please correct me if I'm wrong. Thanks!

class SuperClass {
    var ID = 2202
}

//Wrong
class SubClass: SuperClass {
    override var ID = 2203
}

//worng
class SubClass: SuperClass {
    override var ID: Int {
        return super.ID + 1
    }
}

//Correct
class SubClass: SuperClass {
    override var ID: Int {
        get {
            return super.ID + 1
        }
        set {

        }
    }
}

Your superclass states that objects of it contains a _mutable_ property `ID`. Since suberclass objects are interchangeable with subclass objects, subclass objects must contain a property `ID` that is _mutable_ (even if we use a "dummy" setter as in your example above: not actually mutating the value). This is obvious if you attempt to pass a subclass object to e.g. `func foo(bar: SuperClass) { bar.ID = 42 }`; if the subclass would be allowed to override `ID` with only a getter, then calling `foo(...)` with such objects would not make much sense. — dfrib, Jun 26 '16 at 20:21
... see also the following Q&A:s: [Overriding a stored property in Swift](http://stackoverflow.com/questions/26691935/overriding-a-stored-property-in-swift), [Overriding superclass property with different type in Swift](http://stackoverflow.com/questions/24094158/overriding-superclass-property-with-different-type-in-swift). — dfrib, Jun 26 '16 at 20:25

score 1 · Accepted Answer · answered Jun 26 '16 at 21:42

You cannot override a stored property of the superclass, but you can change its value from the subclass:

class SuperClass {
    var ID = 2202
}

class SubClass: SuperClass {
    override init() {
        super.init()
        self.ID = 2203
    }
}

let a = SuperClass()
let b = SubClass()

print(a.ID)
print(b.ID)

You can only use the computed variable with both setter and getter to overriding the stored property of the superclass

1 Answers1