return nil or something similar in method stub

Question

I am taking a data structures in java course and for fun and learning I am trying to write the stuff in Swift. I am trying to implement a protocol but I am having trouble setting up the method stubs. I tried returning nil but that didn't work but now I am getting this error:

"Swift Compiler Error 'E' is not convertible to 'E'"

That is strange. This is code for a generic array based list. This is what I have so far:

struct ArrayLinearList<E>: LinearListADT {

    let DEFAULT_MAX_SIZE = 100;
    var currentSize: Int
    var maxSize: Int
    var storage = [E]()

    init(sizeOfList: Int) {
        currentSize = 0
        maxSize = sizeOfList
        storage = [E]()
    }


    mutating func addFirst<E>(obj: E) {

    }

    mutating func addLast<E>(obj: E) {

    }

    mutating func insert<E>(obj: E, location: Int) {

    }

    mutating func remove<E>(location: Int) -> E {
        return storage[location] //***This is where I get the above error
    }

    mutating func remove<E>(obj: E) -> E {
        return nil   //I tried this but that didn't work either
    }

    mutating func removeFirst<E>() -> E? {
        return nil   //I also tried this but that didn't work
    }

    mutating func removeLast<E>() -> E? {
        return nil
    }

    mutating func get<E>(location: Int) -> E? {
        return nil
    }

    mutating func contains<E>(obj: E) -> Bool {
        return false
    }

    mutating func locate<E>(obj: E) -> Int? {
        return nil
    }

    mutating func clear<E>() {

    }

    mutating func isEmpty<E>() -> Bool {

    }

    mutating func size<E>() -> Int {

    }

}

EDIT: I just found the mistake. Using the suggestion from Jesper I then found out that I did not write the protocol properly in Swift. Looking at this answer"

how to create generic protocols in swift iOS?

I was able to get it working now. Thank you Jesper!

LinearListADT is the protocol that just contains the method signatures — Guillermo Alvarez, Mar 20 '15 at 13:41

score 1 · Accepted Answer · answered Mar 20 '15 at 13:32

1

You shouldn't have a type parameter E on those methods - it will be considered a separate type parameter from the one on the struct. Remove the <E> in those method definitions and the one from the struct itself will be used.

In addition, you may have to add a constraint to E so that you are sure that it implements NilLiteralConvertible (like an Optional), otherwise you can't return nil from a function that is supposed to return a E.

answered Mar 20 '15 at 13:32

Jesper

7,477
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As an addition to this answer, i think you need to: return storage[location] as E – tskulbru Mar 20 '15 at 13:35
Thank you, there reason why I added them is in the protocol I tried writing this: protocol LinearListADT { and I got an error so that is why I typed it in every method. What modifications should I make to the protocol? – Guillermo Alvarez Mar 20 '15 at 13:36
I don't think so, since `storage` is an array of `E`. – Jesper Mar 20 '15 at 13:36
@Jesper yea, but my compiler complained until i added "as E" – tskulbru Mar 20 '15 at 13:38

score 0 · Answer 2 · edited May 23 '17 at 12:12

0

I just found the mistake. Using the suggestion from Jesper I then found out that I did not write the protocol properly in Swift. Looking at this answer"

How to create generic protocols in Swift?

I was able to get it working now. Thank you Jesper!

edited May 23 '17 at 12:12

Community

1
1

answered Mar 20 '15 at 13:43

Guillermo Alvarez

1,695
2
18
23

return nil or something similar in method stub

2 Answers2