Define struct that is treated like a class in Swift

Question

In Swift a String structure is also treated as a class object like when using the NSCoder encodeObject(_:forKey:) method. I do know that String is directly bridged with the objective-c class, NSString, but is there a way to make a custom struct that behaves similarly? Perhaps bridge it to a custom class? I want to be able to do something like this:

struct SortedArray <Value: Comparable> {}

// Would I need to create a bridge between 
// SortedArray and NSSortedArray? Can I even do that?
class NSSortedArray <Value: Comparable> : NSObject, NSCoding {
    required init?(coder aDecoder: NSCoder) {}
    func encodeWithCoder(aCoder: NSCoder) {}
}

class MyClass : NSObject, NSCoding {
    private var objects: SortedArray<String> = SortedArray<String>()
    required init?(coder aDecoder: NSCoder) {
        guard let objects = aDecoder.decodeObjectForKey("objects") as? SortedArray<String> else { return nil }
        self.objects = objects
    }
    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeObject(objects, forKey: "objects")
    }
}

Answer 1

This isn't currently possible. SE-0058 will address it, but is deferred out of Swift 3. A final implementation of SE-0058 would be hoped to handle more than just ObjC bridging; for example allowing C++ or .NET bridging as well in a more generic solution.

Answer 2

Ultimately, the bridging between String and NSString is quite simple.

NSString only has 2 instance variables (The string pointer nxcsptr, and the length nxcslen). String uses _StringCore, which only has 3 properties (_baseAddress, _countAndFlags, and _owner). The conversion back and forth is hard coded, and called explicitly by the compiler. There's no automatic system implemented for generating classes out of structs, or vice versa.

You'll have to implement a struct/class pair (like with String and NSString), and implement initializers that construct one from the other.