swiftcocoacocoa-touchtoll-free-bridging

Are Swift's String, Array, and Dictionary NSObjects? Why can a struct be an NSObject?

I use NSKeyedArchiver.archivedDataWithRootObject(obj) to transform an object to NSData. The archivedDataWithRootObject(obj) method require its parameter to be an NSObject, conforming to NSCoding.

I tried archiving Swift Strings, Arrays, and Dictionarys, and it worked well. So I think String is an NSObject conforming to NSCoding.

I also checked this code in a playground, to confirm that String is an NSObject:

var str = "Hello, playground"
let isObject = (str is NSObject) // isObject is true

But when I navigate to String's definition (with Cmd + Click), it shows that String is a struct. I cannot find the code showing that String is an NSObject.

public struct String {
    /// An empty `String`.
    public init()
}

So, why in the String definition, can't I find the code showing that String is an NSObject? And why can a struct be an NSObject?

Solution

  • String, Array, Dictionary are all bridged to their Objective-C counterparts (NSString, NSArray and NSDictionary) and can seamlessly act like so.

    String itself does not inherit from NSObject and is actually a struct but it is bridged from NSString which does. When you use a (Swift) String in your code it can act like an NSString thus giving you the output from the code you provided.