Why does the following swift code gives error?

Question

The array does not append with a custom class object

// Create an array with String and Double
var shoppingList = ["Eggs", 2.0]

// Append array with string object
shoppingList += ["Milk"]

// Declare an example class 
    class Foo  {
        var name : String?
        func Foo() {
            name = "Default Name"
        }
}


var foo : Foo = Foo()
shoppingList += [foo] // Error : '[NSObject]' is not identical to 'Uint8'

Why would shoppingList not append foo object?

Its written in the last line. // Error : '[NSObject]' is not identical to 'Uint8' — Vebz, Feb 25 '15 at 17:03
Why is what I am trying to understand. There is no explicit type declared as I was able to add String Type and Int Type. Why would it try to compare the new appended object with Uint8? — Vebz, Feb 25 '15 at 17:06
You got an upvote from me - just because it's a simple question doesn't make it a bad one. That compiler error doesn't help at all, and to a beginner, it's probably not readily apparent that arrays can have automatically-inferred generic type parameters. — Craig Otis, Feb 25 '15 at 18:19
Thanks. It would have been better if everyone could share that perspective. — Vebz, Feb 26 '15 at 11:32

score 1 · Accepted Answer · answered Feb 25 '15 at 18:13

Swift error messages are frequently very cryptic. I'm not sure what the Uint8 is all about.

Swift arrays hold a single type. In this case, your shoppingList has been inferred to be of type [NSObject] since Swift is able to bridge "Eggs" to NSString and 2.0 to NSNumber. Both NSString and NSNumber are subclasses of NSObject, so Swift infers your array to be of type [NSObject].

If you want to put Foo() into that array, then Foo will also need to be a subclass of NSObject:

class Foo: NSObject { ...

Thanks for crystal clear explanation. I tried that and now it makes sense. :) — Vebz, Feb 26 '15 at 11:33

score 1 · Answer 2 · answered Feb 25 '15 at 18:27

Alternatively to @vacawama's solution, you can explicitly specify the array element type rather than letting type inference set it for you:

var shoppingList: [AnyObject] = ["Eggs", 2.0]

The clear advantage of having the value type set to AnyObject is that you don't have to make custom classes inherit from NSObject

Why does the following swift code gives error?

2 Answers2