properties - Swift 类 : Property not initialized at super. init 调用中出现错误

Question

我有两个类，Shape 和 Square

class Shape {
    var numberOfSides = 0
    var name: String
    init(name:String) {
        self.name = name
    }
    func simpleDescription() -> String {
        return "A shape with \(numberOfSides) sides."
    }
}

class Square: Shape {
    var sideLength: Double

    init(sideLength:Double, name:String) {
        super.init(name:name) // Error here
        self.sideLength = sideLength
        numberOfSides = 4
    }
    func area () -> Double {
        return sideLength * sideLength
    }
}

通过上面的实现，我收到错误:

property 'self.sideLength' not initialized at super.init call
    super.init(name:name)

为什么我必须在调用 super.init 之前设置 self.sideLength？

Answer 1

引用《Swift 编程语言》，它回答了您的问题:

“Swift’s compiler performs four helpful safety-checks to make sure that two-phase initialization is completed without error:”

Safety check 1 “A designated initializer must ensure that all of the “properties introduced by its class are initialized before it delegates up to a superclass initializer.”

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itunes.apple.com/us/book/swift-programming-language/id881256329?mt=11