How to store custom objects in NSUserDefaults

On your Player class, implement the following two methods (substituting calls to encodeObject with something relevant to your own object):

- (void)encodeWithCoder:(NSCoder *)encoder {
    //Encode properties, other class variables, etc
    [encoder encodeObject:self.question forKey:@"question"];
    [encoder encodeObject:self.categoryName forKey:@"category"];
    [encoder encodeObject:self.subCategoryName forKey:@"subcategory"];
}

- (id)initWithCoder:(NSCoder *)decoder {
    if((self = [super init])) {
        //decode properties, other class vars
        self.question = [decoder decodeObjectForKey:@"question"];
        self.categoryName = [decoder decodeObjectForKey:@"category"];
        self.subCategoryName = [decoder decodeObjectForKey:@"subcategory"];
    }
    return self;
}

Reading and writing from NSUserDefaults:

- (void)saveCustomObject:(MyObject *)object key:(NSString *)key {
    NSData *encodedObject = [NSKeyedArchiver archivedDataWithRootObject:object];
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    [defaults setObject:encodedObject forKey:key];
    [defaults synchronize];

}

- (MyObject *)loadCustomObjectWithKey:(NSString *)key {
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSData *encodedObject = [defaults objectForKey:key];
    MyObject *object = [NSKeyedUnarchiver unarchiveObjectWithData:encodedObject];
    return object;
}

Code shamelessly borrowed from: saving class in nsuserdefaults

Swift 4 introduced the Codable protocol which does all the magic for these kinds of tasks. Just conform your custom struct/class to it:

struct Player: Codable {
  let name: String
  let life: Double
}

And for storing in the Defaults you can use the PropertyListEncoder/Decoder:

let player = Player(name: "Jim", life: 3.14)
UserDefaults.standard.set(try! PropertyListEncoder().encode(player), forKey: kPlayerDefaultsKey)

let storedObject: Data = UserDefaults.standard.object(forKey: kPlayerDefaultsKey) as! Data
let storedPlayer: Player = try! PropertyListDecoder().decode(Player.self, from: storedObject)

It will work like that for arrays and other container classes of such objects too:

try! PropertyListDecoder().decode([Player].self, from: storedArray)