问题描述

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，我得到了错误:

With the implementation above I get the error:

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?

Why do I have to set self.sideLength before calling super.init?

推荐答案

引自 The Swift Programming Language，它回答了你的问题:

Quote from The Swift Programming Language, which answers your question:

Swift 的编译器会执行四项有用的安全检查，以确保两阶段初始化完成，没有错误:"

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

安全检查 1 指定的初始化程序必须确保所有由它的类引入的属性在它之前被初始化委托给一个超类初始化器."

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."

摘自:Apple Inc. Swift 编程语言".电子书.https://itunes.apple.com/us/book/swift-programming-language/id881256329?mt=11

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

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