mutating method sent to immutable object

Question

When I use this method first time it works fine, but when I called it second time I get the error \"mutating method sent to immutable object\". The problem is at line with \"add

Answer 1

As the documentation for NSUserDefaults says: "Values returned from NSUserDefaults are immutable, even if you set a mutable object as the value." Whenever you want to change a collection you get from NSUserDefaults you have to get the immutable version, make a mutableCopy, modify that, and set it back again.