NSUserDefaults で struct を保存（？）する方法 ref: http://qiita.com/lovee/items/2b39e1c54d6d00adead9

file0.swift

class ViewController: UIViewController {

    var settings: UserSettings

    init() {
        self.settings = UserSettings(a: 0, b: false, c: "NO")

        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}
struct UserSettings {

    var a: Int
    var b: Bool
    var c: String

}

file1.swift

struct UserSettings {

    var a: Int
    var b: Bool
    var c: String

}

class UserSettingsObject {
    var a: Int
    var b: Bool
    var c: String

    // すでに設定があれば流用します
    init(settings: UserSettings) {
        self.a = settings.a
        self.b = settings.b
        self.c = settings.c
    }

    // まだ設定が作られていない場合はデフォルト設定を作ります
    init() {
        self.a = 0
        self.b = false
        self.c = "NO"
    }
}

class ViewController: UIViewController {

    let userDefaults = NSUserDefaults.standardUserDefaults()
    var settings: UserSettings

    init() {

        let settingsObject = self.userDefaults.valueForKey("SaveData") as? UserSettingsObject ?? UserSettingsObject()
        self.settings = UserSettings(a: settingsObject.a, b: settingsObject.b, c: settingsObject.c)

        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func save() {
        let settingObject = UserSettingsObject(settings: self.settings)
        self.userDefaults.setValue(settingObject, forKey: "SaveData")
    }

}

file2.swift

struct UserSettings {

    var a: Int
    var b: Bool
    var c: String

    static func userSettingsFromNSDictionary(dictionary: NSDictionary?) -> UserSettings {
        let aValue = dictionary?["a"] as? Int ?? 0
        let bValue = dictionary?["b"] as? Bool ?? false
        let cValue = dictionary?["c"] as? String ?? "NO"

        return UserSettings(a: aValue, b: bValue, c: cValue)
    }

    func nsdictionaryForNSUserDefaults() -> NSDictionary {
        let dictionary: NSDictionary = [
            "a": self.a,
            "b": self.b,
            "c": self.c,
        ]
        return dictionary
    }

}

class ViewController: UIViewController {

    let userDefaults = NSUserDefaults.standardUserDefaults()
    var settings: UserSettings

    init() {

        let settingsDictionary = self.userDefaults.valueForKey("SaveData") as? NSDictionary
        self.settings = UserSettings.userSettingsFromNSDictionary(settingsDictionary)

        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func save() {
        let settingDictionary = self.settings.nsdictionaryForNSUserDefaults()
        self.userDefaults.setValue(settingDictionary, forKey: "SaveData")
    }

}