When writing a struct, class, or enum it may be helpful to divide the code into several different sections. By using an extension, you can continue writing the definition of a struct, class, or enum anywhere in your codebase.

struct Cat {
    let name: String
    let age: Int
}

extension Cat {
    static let famousCats = [
        Cat(name: "Stubbs", age: 20), 
        Cat(name: "Sweet Tart", age: 12), 
        Cat(name: "Hank the Cat", age: 13)
    ]
}

print(Cat.famousCats.count) // prints 3

Code that’s written in an extension will behave exactly the same as if it was defined in the original struct. You can even extend structs, classes, and enums from the Swift Standard Library!

extension Int {
    var isEven: Bool {
        isMultiple(of: 2)
    }
}

let catCount = 3
print(catCount.isEven) // prints false

The only thing that you can’t do in extensions is create stored properties:

struct Cat {
    let name: String
    let age: Int
}

extension Cat {
    var numberOfLegs: Int // ERROR: Extensions must not contain stored properties
}

This restriction ensures that adding an extension will never cause your code to not compile by adding new requirements. Instead, you can add computed properties:

struct Cat {
    let name: String
    let age: Int
}

extension Cat {
    var numberOfLegs: Int { return 4 }
}

Let’s try adding an extension to our Office!