Is String type a class or a struct? Or something else?

Question

class MyString: String {}

gives an error:

Inheritance from non-protocol, non-class type 'String'`.

So looks like String is not a class. But why it can be used in context where AnyObject is expected? I though AnyObject is for class type only, while Any could be a class or non-class type.

Answer 1

If you command+click String in Xcode

struct String {
    init()
}

So String is a struct.

You can't subclass from struct as the error message said:

error: Inheritance from non-protocol, non-class type 'String'

However, you can implicitly convert it to NSString (which is subtype of AnyObject) only if you have imported Foundation directly or indirectly.

From REPL

  1> var str = "This is Swift String" 
str: String = "This is Swift String"
  2> var anyObj : AnyObject = str 
.../expr.oXbYls.swift:2:26: error: type 'String' does not conform to protocol 'AnyObject'
var anyObj : AnyObject = str
                         ^

---

  1> import Foundation // <---- always imported in any useful application
  2> var str = "This is Swift String" 
str: String = "This is Swift String"
  3> var anyObj : AnyObject = str 
anyObj: _NSContiguousString = "This is Swift String" // it is not Swift string anymore
  4>  

---

But try to avoid to subclass String or NSString unless you absolute have to. You can read more at NSString Subclassing Notes.

Use extension to add new method to String

extension String {
    func print() {
        println(self);
    }
}

"test".print()

Use delegation pattern if you want to have more control over it

struct MyString {
    let str : String = ""

    func print() {
        println(str)
    }
}