Structs, Classes and Protocols

Structs are value types

A struct groups related data and behaviour, and it is a value type: copying it makes an independent copy. Swift favours structs for most models.

struct Point {
    var x: Int
    var y: Int
    func distanceToOrigin() -> Double {
        Double((x*x + y*y)).squareRoot()
    }
}
var a = Point(x: 3, y: 4)
var b = a          // a full copy
b.x = 99           // does NOT change a

Classes are reference types

A class looks similar but is a reference type: copies share the same underlying object, and classes support inheritance. Use classes when you need shared, mutable state or identity.

Protocols: contracts

A protocol defines a set of requirements a type promises to fulfil – like an interface. Protocol-oriented design is core Swift style:

protocol Describable {
    var summary: String { get }
}
struct Book: Describable {
    let title: String
    var summary: String { "Book: (title)" }
}

Anything that conforms to Describable can be used wherever a Describable is expected.

Key points

• Structs are value types (copied); prefer them for models.
• Classes are reference types (shared) and support inheritance.
• Protocols define capabilities that many types can adopt.
• Swift favours composition with protocols over deep class hierarchies.