Swift Language Guide - Class and Structure

Common

• Define properties to store values
• Define method to provide functionality
• Define subscriptions to provide access to their values using subscript syntax
• Define initializers to set up their initial state
• Be extended to expand their functionality beyond a default implementation
• Confirm to protocols to provide standard functionality of certain kind

Structure NOT

• Inheritance
• Type Casting
• De-initializers
• Reference Counting (Structure is Value Type, Class is Reference Type)

Initialize Syntax

Initialization MUST use External Names

init is with External Names implicitly

Structure

struct Color {
    let red = 0.0, green = 0.0, blue = 0.0
}

let black = Color()
let white = Color(red: 255.0, green: 255.0, blue: 255.0)
let c1 = Color(255.0, 255.0, 255.0) <-- Error, MUST use External Names

struct Color {
    let red = 0.0, green = 0.0, blue = 0.0

    init(red: Double) {     <-- Constructor
        self.red = red
    }
}


let black = Color()     <-- Error

let white = Color(red: 255.0, green: 255.0, blue: 255.0)    <-- Error
let c1 = Color(255.0, 255.0, 255.0) <-- Error
let c2 = Color(255.0)                   <-- Error
let c2 = Color(red: 255.0)              <-- OK, remember to USE External Names

Class

class Color {
    let red = 0.0, green = 0.0, blue = 0.0
}


let black = Color()     <-- OK
let white = Color(red: 255.0, green: 255.0, blue: 255.0)    <-- Error, Constructor is NOT exist.

class Color {
    let red = 0.0, green = 0.0, blue = 0.0

    init(red: Double, green: Double, blue: Double) {
        self.red = red
        self.green = green
        self.blue = blue
    }    
}

let black = Color()     <-- Error, Constructor is NOT exist.
let white = Color(red: 255.0, green: 255.0, blue: 255.0)    <-- OK

class Color {
    let red = 0.0, green = 0.0, blue = 0.0

    init(red: Double, green: Double, blue: Double) {
        self.red = red
        self.green = green
        self.blue = blue
    }

    init() {
        self.red = 255.0
        self.green = 255.0
        self.blue = 255.0
    }
}


let black = Color()     <-- OK
let white = Color(red: 255.0, green: 255.0, blue: 255.0)    <-- OK

Initializer

• Designated initializer : primary initializers. May have MANY.

• Convenience initializer : secondary initializers.

  • Use convenience
  • MUST invoke designated initializer

class Color {
    let red = 0.0, green = 0.0, blue = 0.0

    init(red: Double, green: Double, blue: Double) {
        self.red = red
        self.green = green
        self.blue = blue
    }

    convenience init() {    <-- Error, convenience initializer MUST invoke designated one
        self.red = 255.0
        self.green = 255.0
        self.blue = 255.0
    }
}

class Color {
    let red = 0.0, green = 0.0, blue = 0.0

    init(red: Double, green: Double, blue: Double) {
        self.red = red
        self.green = green
        self.blue = blue
    }

    convenience init() {    <-- OK, convenience initializer invoke a designated one
        self.init(red: 255.0, green: 255.0, blue: 255.0)
    }
}

Inheritance

• Designated and Convenience initializers in subclass MUST invoke Designated one of superclass
• Subclass can NOT override Convenience initializer of superclass to Designated
• Be careful with Initializer Chain Problem in Convenience Initializer of subclass.

Use Cases

• No initializer defined in subclass and use Designated one of superclass

    class Color {
        let red = 0.0, green = 0.0, blue = 0.0
  
        init(red: Double, green: Double, blue: Double) {    <-- Designated
            self.red = red
            self.green = green
            self.blue = blue
        }
    }
  
    class AlphaColor : Color {
        let alpha = 255.0
    }
  
  
    let c1 = AlphaColor(red: 255.0, green: 128.0, blue: 255.0)  <-- Use designated initializer of superclass
    let c2 = AlphaColor()   <-- Error.
  
    println("(\(c1.red), \(c1.green), \(c1.blue), \(c1.alpha))")    <-- (255.0, 128.0, 255.0, 255.0)
  

• No initializer defined in subclass and use Convenience one of superclass

    class Color {
        let red = 0.0, green = 0.0, blue = 0.0
  
        init(red: Double, green: Double, blue: Double) {
            self.red = red
            self.green = green
            self.blue = blue
        }
  
        convenience init() {
            self.init(red: 128.0, green: 128.0, blue: 128.0)
  
        }
  
    }
  
    class AlphaColor : Color {
        let alpha = 255.0
    }
  
    let c1 = AlphaColor()   <-- Use Convenience initializer of superclass
  
    println("(\(c1.red), \(c1.green), \(c1.blue), \(c1.alpha))")    <-- (128.0, 128.0, 128.0, 255.0)
  

• Subclass MUST use Designated initializer of superclass in its initializers.

    class Color {
        let red = 0.0, green = 0.0, blue = 0.0
  
        init(red: Double, green: Double, blue: Double) {
            self.red = red
            self.green = green
            self.blue = blue
        }
  
        convenience init() {
            self.init(red: 128.0, green: 128.0, blue: 128.0)
  
        }
  
    }
  
    class AlphaColor : Color {
        let alpha = 255.0
  
        init(alpha: Double) {
            super.init(red: 0.0, green: 0.0, blue: 0.0)
            self.alpha = alpha
        }
    }
  
    let c1 = AlphaColor(alpha: 128.0)
  
    println("(\(c1.red), \(c1.green), \(c1.blue), \(c1.alpha))")    <-- (0.0, 0.0, 0.0, 128.0)
  

    class Color {
        let red = 0.0
        var green = 0.0, blue = 0.0
  
        init(red: Double, green: Double, blue: Double) {
            self.red = red
            self.green = green
            self.blue = blue
        }
  
        convenience init() {
            self.init(red: 128.0, green: 128.0, blue: 128.0)
  
        }
  
    }
  
    class AlphaColor : Color {
        let alpha = 255.0
  
        init(alpha: Double) {
            super.init()            <-- Error, invoke convience initializers of superclass
            self.alpha = alpha
        }
    }
  

    class Color {
        let red = 0.0
        var green = 0.0, blue = 0.0
  
        init(red: Double, green: Double, blue: Double) {
            self.red = red
            self.green = green
            self.blue = blue
        }
  
        convenience init() {
            self.init(red: 128.0, green: 128.0, blue: 128.0)
  
        }
  
    }
  
    class AlphaColor : Color {
        let alpha = 255.0
  
        init(alpha: Double) {
            super.init(red: 0.0, green: 0.0, blue: 0.0)
            self.alpha = alpha
        }
  
        convenience init() {
            super.init()    <-- Error, Must invoke designdated initializers of superclass
        }
    }