Convenience Init Override

Question

Problem

Override a convenience initializer of a subclass and it produces a compile error.

Detail

I am having issues understanding why Swift (v4.1) is not letting me override my convenience initializer. Reading the documentation I found that these two rules apply to my question:

Rule 1 If your subclass doesn’t define any designated initializers, it automatically inherits all of its superclass designated initializers.

Rule 2 If your subclass provides an implementation of all of its superclass designated initializers—either by inheriting them as per rule 1, or by providing a custom implementation as part of its definition—then it automatically inherits all of the superclass convenience initializers.

In the code below I fall under the first rule and all my convenience initializers are inherited into ClassB. Furthermore, since I inherited all the designated initializers as per rule one, I also get all my convenience initializers inherited.

class ClassA<T> {

    // This array would be private and not visible from ClassB
    var array: [T]?

    init() { }

    convenience init(array: [T]) {
        self.init()

        self.array = array
    }
}

class ClassB<T>: ClassA<T> {

    var anotherArray: [T]?

    // I feel like I should include the "override" keyword
    // but I get a compiler error when "override" is added before "convenience init".
    convenience init(array: [T]) {
        self.init()

        self.anotherArray = array
    }
}

// Works fine
let instanceA = ClassA(array: [1, 2])
// Compile error when override is added:
// error: Initializer does not override a designated initializer from its superclass
// note: attempt to override convenience initializer here
//     convenience init(array: [T]) {
//                 ^
let instanceB = ClassB(array: [1, 2])

But here is what I don't understand: ClassB has a slightly different implementation of init(array:) and I would like to override that convenience initializer. Using the override keyword produces a compile error. Am I understanding these initialization concepts wrongly?

“If the initializer you are overriding is a convenience initializer, your override must call another designated initializer from its own subclass.” — pNre, Aug 15 '14 at 07:36
So I will need to create a new designated initialiser even if I don't need one? I solved my issue by creating an internal function which does the logic the convenience initialiser did. Separating the code that way allowed me to override it correctly. — Jad, Aug 15 '14 at 07:43
That's what the book says, but doesn't look right since `init` is a designated `ClassB` initializer. That could easily be a bug. — pNre, Aug 15 '14 at 07:53

jtbandes · Accepted Answer · 2015-09-17T04:10:32.347

The reason override is unneeded:

Conversely, if you write a subclass initializer that matches a superclass convenience initializer, that superclass convenience initializer can never be called directly by your subclass, as per the rules described above in Initializer Chaining. Therefore, your subclass is not (strictly speaking) providing an override of the superclass initializer. As a result, you do not write the override modifier when providing a matching implementation of a superclass convenience initializer.

But as written, it seems it should work — as far as I can tell this is a compiler bug. If you change the name of the array argument to ClassB's initializer to e.g. array2, then it works as expected. You should file a Radar!

Thanks @jtbandes, I must have missed that bit of the documentation. I will set your answer as the accepted one. — Jad, Aug 15 '14 at 07:57

Convenience Init Override

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