Type erasure concept in Swift - Mini Project: Build & Apply

Understanding Type Erasure in Swift

📖 Scenario: Imagine you are building a simple app that manages different types of shapes. Each shape can calculate its area, but they have different types. You want to store them together and work with them uniformly.

🎯 Goal: You will create a protocol for shapes, implement two different shapes, then use type erasure to store them in a single array and calculate their total area.

📋 What You'll Learn

Create a protocol called Shape with a method area() returning Double

Create two structs Circle and Rectangle conforming to Shape

Create a type-erased wrapper called AnyShape to hold any Shape

Store multiple AnyShape instances in an array and calculate the total area

💡 Why This Matters

🌍 Real World

Type erasure helps when you want to work with different types that share behavior but cannot be stored together directly, such as UI components or data models.

💼 Career

Understanding type erasure is important for Swift developers to write flexible and reusable code, especially when working with protocols and generics.

1

Create the Shape protocol and two shape structs

Create a protocol called Shape with a method area() that returns a Double. Then create two structs: Circle with a radius property, and Rectangle with width and height properties. Both structs must conform to Shape and implement the area() method.

Swift

# Your code here

Need a hint?

Protocols define what methods a type must have. Structs can conform to protocols by implementing those methods.

2

Create the AnyShape type-erased wrapper

Create a struct called AnyShape that conforms to Shape. It should have a private property holding a closure for the area() method. Add an initializer that accepts any Shape and stores its area() method in the closure.

Swift

protocol Shape {
    func area() -> Double
}

struct Circle: Shape {
    var radius: Double
    func area() -> Double {
        return Double.pi * radius * radius
    }
}

struct Rectangle: Shape {
    var width: Double
    var height: Double
    func area() -> Double {
        return width * height
    }
}

// Your code here

Need a hint?

Type erasure uses a wrapper that hides the specific type but keeps the behavior by storing closures.

3

Create an array of AnyShape and calculate total area

Create an array called shapes of type [AnyShape]. Add a Circle with radius 3 and a Rectangle with width 4 and height 5, wrapped as AnyShape. Then create a variable totalArea and use a for loop with variables shape to sum all areas from shapes.

Swift

protocol Shape {
    func area() -> Double
}

struct Circle: Shape {
    var radius: Double
    func area() -> Double {
        return Double.pi * radius * radius
    }
}

struct Rectangle: Shape {
    var width: Double
    var height: Double
    func area() -> Double {
        return width * height
    }
}

struct AnyShape: Shape {
    private let _area: () -> Double
    
    init(_ shape: S) {
        self._area = shape.area
    }
    
    func area() -> Double {
        return _area()
    }
}

// Your code here

Need a hint?

Wrap each shape in AnyShape to store them together. Use a loop to add their areas.

4

Print the total area

Write a print statement to display the text Total area: followed by the value of totalArea.

Swift

protocol Shape {
    func area() -> Double
}

struct Circle: Shape {
    var radius: Double
    func area() -> Double {
        return Double.pi * radius * radius
    }
}

struct Rectangle: Shape {
    var width: Double
    var height: Double
    func area() -> Double {
        return width * height
    }
}

struct AnyShape: Shape {
    private let _area: () -> Double
    
    init(_ shape: S) {
        self._area = shape.area
    }
    
    func area() -> Double {
        return _area()
    }
}

var shapes: [AnyShape] = [AnyShape(Circle(radius: 3)), AnyShape(Rectangle(width: 4, height: 5))]
var totalArea = 0.0
for shape in shapes {
    totalArea += shape.area()
}

// Your code here

Need a hint?

Use string interpolation with print("Total area: \(totalArea)") to show the result.