Mutation method (mutating)

struct Menu {
  var menuItems = ["Fries", "Burgers"]
  mutating func addToMenu(dish: String) {
    self.menuItems.append(dish)
  }
}

Using the Menu class

var dinerMenu = Menu()
dinerMenu.addToMenu(dish: "Toast")
print(dinerMenu.menuItems)
// prints: ["Fries", "Burgers", "Toast"]