How to set default values in Swift compiler-generated initializers

For the longest time, I used to roll out my own init when I wanted to set default values for a let property of a struct. Today, I discovered a way to retain the compiler-generated init and have it set the default value, too.

One of the many cool Swift compiler features that save development time is the compiler-generate initializer for structs. Unless you explicitly define an init method, the compiler will generate one for you based on the properties of your struct.

Consider this Pizza struct:

struct Pizza {

  let name: String
  let ingredients: [Ingredient]
}

The compiler will generate an init method for it with the parameters in the same order as the properties definition.

let pizza = Pizza(name: "Margherita", ingredients: [.tomatoSauce, .mozzarella])

Now, imagine we want to add extras to our Pizza. By default, a pizza should not have extras, but consumers can decide to add some – I like my Margherita extra cheesy, so I always ask for double mozzarella.

One way to do that is to define an init with the default value:

struct Pizza {

  let name: String
  let ingredients: [Ingredient]
  let extra: [Ingredient]

  init(name: String, ingredients: [Ingredient], extras: [Ingredient] = []) {
    self.name = name
    self.ingredients = ingredients
    self.extras = extras
  }
}

Unfortunately, by providing our own init, we take responsibility of updating it every time the shape of the struct changes.

A different way to have a default value while also leveraging the compiler-generated init is to set it as part of the property definition.

struct Pizza {

  let name: String
  let ingredients: [Ingredient]
  private(set) var extras: [Ingredient] = []
}

It never occurred to me to do this because I try to keep my code as immutable as I can; using a var leaves the door open for mutations.

On the other hand, the private(set) access control mitigates the mutable state risk: only the code internal to the struct can change the value of extras.

That seems like a good tradeoff. I can keep the default value, the compiler-generate init, and make the struct immutable for consumers. I only need to be mindful not to change the property in the struct implementation.

What do you think of this little trick / feature? Leave a comment below or get in touch on Twitter at @mokagio.

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