使用引數自定義初始化

struct MetricDistance {
    var distanceInMeters: Double

    init(fromCentimeters centimeters: Double) {
        distanceInMeters = centimeters / 100
    }
    init(fromKilometers kilos: Double) {
        distanceInMeters = kilos * 1000
    }
}

let myDistance = MetricDistance(fromCentimeters: 42)
// myDistance.distanceInMeters is 0.42
let myOtherDistance = MetricDistance(fromKilometers: 42)
// myOtherDistance.distanceInMeters is 42000

請注意，你不能省略引數標籤：

let myBadDistance = MetricDistance(42) // error: argument labels do not match any available overloads

為了省略引數標籤，請使用下劃線 _ 作為標籤：

struct MetricDistance {
    var distanceInMeters: Double
    init(_ meters: Double) {
        distanceInMeters = meters
    }
}
let myDistance = MetricDistance(42) // distanceInMeters = 42

如果引數標籤與一個或多個屬性共享名稱，請使用 self 顯式設定屬性值：

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