Structures and Classes

Structures and classes allow you to create custom types that bundle data and behavior.

Similarities

Both can:

• Store values in properties
• Define methods for functionality
• Use subscripts for value access
• Set initial state with initializers
• Be extended for added features
• Conform to protocols for standard behavior

Differences: Classes Only

Classes offer extras that structures lack:

• Inheritance from other classes
• Runtime type casting
• Deinitializers for cleanup
• Reference counting for shared instances

Tip: Use structures by default for simplicity; opt for classes when needed.

Definition Syntax

Use struct for structures and class for classes:

struct Point {
    var x = 0
    var y = 0
}

class Person {
    var name: String = "Unknown"
    var age = 0
}

Naming: Types in UpperCamelCase; properties/methods in lowerCamelCase.

Instances

Create with parentheses:

let origin = Point()
var teacher = Person()

Property Access

Use dot syntax:

print("Point at (\(origin.x), \(origin.y))")  // Point at (0, 0)

teacher.name = "Alice"
teacher.age = 28

print("\(teacher.name), age \(teacher.age)")  // Alice, age 28

Memberwise Initializers (Structures)

Structures get them automatically:

let customPoint = Point(x: 5, y: 10)

Classes do not.

Value Types: Structures and Enumerations

Value types copy on assignment:

var p1 = Point(x: 3, y: 4)
var p2 = p1  // Copy made

p2.x = 7

print(p1.x)  // 3
print(p2.x)  // 7

Enum example:

enum Direction {
    case up, down, left, right
    
    mutating func turnLeft() {
        switch self {
        case .up:    self = .left
        case .left:  self = .down
        case .down:  self = .right
        case .right: self = .up
        }
    }
}

var heading = Direction.right
let savedHeading = heading

heading.turnLeft()

print(heading)     // up
print(savedHeading)  // right

Reference Types: Classes

Classes share instances:

let employee1 = Person()
employee1.name = "Bob"
employee1.age = 30

let employee2 = employee1  // Same instance
employee2.age = 31

print(employee1.age)  // 31
print(employee2.age)  // 31

Identity Check (Classes)

Use === / !==:

if employee1 === employee2 {
    print("Same instance")
}  // Same instance

=== checks identity; == checks equality (implement if needed).

Summary Table

Aspect	Structure	Class
Type	Value	Reference
On Assignment	Copied	Shared
Initializer	Memberwise	Custom
Inheritance	No	Yes
Deinitializers	No	Yes

Prefer: Structures for most cases.