Pass Array containing either class or struct

Question

In Swift I declared a function that differs from Array.count only in that if array == nil the function returns 0. This is related to my UITableViewDataSource, but that's not important here. The problem is, if I declare the function as:

class func countOfItemsInArray(array: [AnyObject]?) -> Int

and then try to pass it an array of structs, it declares that the structs in the array do not conform to AnyObject. I understand why that is (I think), but is there a way to make this work with classes and structs, or should I just give in to copy and paste?

Airspeed Velocity · Accepted Answer · 2015-01-31 20:39:23Z

3

Generics are probably better suited to this problem than relying on covariance of [AnyObject]. A version of countElements that worked on an optional array and returned 0 in case of nil could go like this:

func countElements<T>(array: [T]?) -> Int {
    return array?.count ?? 0
}

When you call countElements with any kind of array, the placeholder T is replaced with the type of the element contained in the array.

Note, this version overloads the existing countElements with a version that takes an optional. If you call it with a non-optional or any other kind of collection, the Swift version would be called, if you pass in an optional array, this one will be called. It’s debatable whether this is a good practice (I think it’s fine) or a bad one (some may disapprove :).

A version that works on any collection type would be:

func countElements<C: CollectionType>(col: C?) -> C.Index.Distance {
    return col.map { countElements($0) } ?? 0
}

edited Jan 31, 2015 at 20:39

answered Jan 31, 2015 at 19:14

Airspeed Velocity

41.1k8 gold badges116 silver badges119 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Luca Angeletti Over a year ago

Your solution with generics is perfect. However I am among those who would avoid overriding a Swift standard function. More specifically I admit I don't know if Cocoa (and other OS libraries/frameworks) can see your version of countElements. If the can maybe the override approach is a little insecure.

Airspeed Velocity Over a year ago

There’s no risk from Cocoa (even if it was a possibility which I suspect it is not) calling this version, since the the Swift version comes ahead of it in the overload resolution priority (even though Swift will implicitly upconvert non-optionals to optionals, calls that require that appear to come lower in the pecking order). I would not advocate overloading an existing function in a way that would cause ambiguity for existing callers (though I guess you could do it unwittingly).

Luca Angeletti Over a year ago

Ok in your declaration you did not use the "public" access control modifier. So your version of countElements is visibile only inside the project where it is declared. Then it should not interfere with other library.

Marcus Rossel · Accepted Answer · 2015-01-31 19:18:10Z

0

If you use Any instead of AnyObject you can pass any type, so also structs:

class func countOfItemsInArray(array: [Any]?) -> Int

This is kind of weird.

I used this function:

func countOfItemsInArray(array: [Any]?) -> Int {
    return array != nil ? array!.count : 0
}

Declared two of your Assignment structs and put them in an array:

let structOne = Assignment(name: "1", dueDate: NSDate(), subject: "1")
let structTwo = Assignment(name: "2", dueDate: NSDate(), subject: "2")
let myArray: [Assignment] = [structOne, structTwo]

But here's the interesting part.
When calling println(countOfItemsInArray(myArray)) it gives the error:

<stdin>:27:33: error: 'Assignment' is not identical to 'Any'
println(countOfItemsInArray(myArray))
^
<stdin>:17:26: note: in initialization of parameter 'array'
func countOfItemsInArray(array: [Any]?) -> Int {
^

So I tested if myArray is of type [Any]:

println(myArray is [Any])

to which swift says:

<stdin>:25:17: error: 'Any' is not a subtype of 'Assignment'
println(myArray is [Any])
^

But when I change the type annotation of myArray to [Any] it works:

let myArray: [Any] = [structOne, structTwo]

And when simply handing the literal to the function it works, too:

countOfItemsInArray([structOne, structTwo])

The whole code example can be seen here.

edited Jan 31, 2015 at 19:18

answered Jan 31, 2015 at 18:37

Marcus Rossel

3,3201 gold badge29 silver badges46 bronze badges

1 Comment

PopKernel Over a year ago

I'm getting an error, "'myStructType' is not identical to 'Any'"

Collectives™ on Stack Overflow

Pass Array containing either class or struct

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related