Nullable 和 Non Nullable 型別

普通型別，如 String，不可為空。為了使它們能夠儲存空值，你必須通過在它們後面加上 ? 來明確表示：String?

var string        : String = "Hello World!"
var nullableString: String? = null

string = nullableString    // Compiler error: Can't assign nullable to non-nullable type.
nullableString = string    // This will work however!