Structs & Classes

Structs and classes are both building blocks in Swift used to create custom data types. You use them to group related variables and functions together.

Commonalities

Both can:

• Define properties to store values.
• Define methods to provide functionality.
• Define initializers to set up their initial state.
• Be extended to expand their functionality (Explained later).

Properties

Properties are simply variables or constants associated with a particular class or struct. Just like global variables, they can be Stored or Computed.

struct Circle {
    var radius: Double // Stored Property
    
    var area: Double { // Computed Property
        3.14159 * radius * radius // Implicit return
    }
}

The Key Differences: Struct vs Class

Feature	Structs	Classes
Type Category	Value Type: Stored directly.	Reference Type: Stored as a pointer.
Storage	Copied: Each variable gets its own unique copy of the data.	Shared: Multiple variables point to the exact same instance.
Inheritance	❌ No inheritance support.	✅ Can inherit from other classes.
Initializers (init)	🪄 Gets a Memberwise Initializer automatically.	✍️ You must write your own initializers.
Mutability	🔒 If the struct is let, you cannot change its properties.	🔓 If the class is let, you can still change its var properties.
Cleanup (deinit)	❌ No deinitializers.	✅ Supports deinit for cleanup right before it is destroyed.
Identity Check (===)	❌ Not available (value types have no identity).	✅ Classes Only: Use === to check if same instance.
Memory	⚡ Fast: Stored on the Stack.	🐢 Slower: Stored on the Heap.
Use Case	Default Choice. Use for 90% of things (models, state).	Use for complex objects with shared state or inheritance.

NOTE

Memberwise Initializers: A "magic" feature where the Swift compiler automatically generates an initializer for structs based on their properties. Classes do not get this.

Struct Example (Value Type)

struct Resolution {
    var width = 0
    var height = 0
}

let hd = Resolution(width: 1920, height: 1080) // Memberwise Initializer: no manual 'init' needed
var cinema = hd      // 📦 A COPY is created here
cinema.width = 2048  // Only 'cinema' changes; 'hd' remains 1920

Class Example (Reference Type)

class VideoMode {
    var resolution: Resolution
    var frameRate: Double

    // Classes need manual initializers
    init(resolution: Resolution, frameRate: Double) {
        self.resolution = resolution
        self.frameRate = frameRate
    }
}

let tenEighty = VideoMode(resolution: hd, frameRate: 60.0)
let alsoTenEighty = tenEighty  // 🔗 Both point to the SAME instance
alsoTenEighty.frameRate = 30.0 // Changing 'alsoTenEighty' also changes 'tenEighty'

When to use which?

• Use Structs by default. They are safer and faster.
• Use Classes when you need inheritance or shared identity (reference types).