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
}
}
使用上面的实现我得到错误：
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？
报价从Swift编程语言，它回答你的问题：
“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. 07000