属性 – Swift类中的错误：属性未在super.init调用初始化

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？