使用参数自定义初始化

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

请注意，你不能省略参数标签：

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

为了省略参数标签，请使用下划线 _ 作为标签：

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

如果参数标签与一个或多个属性共享名称，请使用 self 显式设置属性值：

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