Class Fields in Swift

Full lesson:

Right now your Dog class is just an empty shell — every Dog object you create is identical to every other one. We need a way to attach data to each object.

That's what fields are for. A field is a variable that belongs to a class — and each object created from that class gets its own copy of every field.

Declaring fields

class Dog {
    var name: String = ""
    var age: Int = 0
}

• var declares a mutable field — any code can read and write it. You'll use let later for fields that should not change after being set.

• name: String is just a variable declaration — same syntax you'd use anywhere else.

Reading and writing fields

var dog = Dog()
dog.name = "Rex"        // write
dog.age = 4
print(dog.name)   // read → prints "Rex"

Each object has its own slot for every field, so two dogs can have different names without interfering.

Default values

In Swift, every stored property must have a value by the time an object is created, and numbers, Bool and String properties do not get one automatically. Give each a default value in the declaration, or set it in an initializer (a later lesson). Typical defaults are:

• Numbers: 0

• Bool: false

• String: ""

• An optional property such as var nickname: String? starts as nil without an explicit default

Your task

Update the Dog class to have two fields: name (a String) and age (an Int). Then in mainFunction, set them on the dog object so the pre-filled print prints:

Rex is 4 years old