为什么 Swift 需要在 super.init() 之前初始化子类的属性

Question

问题来了，我想知道 init 是如何工作的，子类的属性存储在哪里

Answer 1

Here's an example, to exemplify one of the reasons, let's say you have the following class definitions:

class Base {
    init() {
        sayHello()
    }

    func sayHello() {
        print("Hello from base")
    }
}

class Derived: Base {
    let name: String

    override init() {
        // no initialization of own properties, just call super()
        super.init()
    }

    override func sayHello() {
        print("Hello from \(name)")
    }
}

Basically, the initialized of the base class calls sayHello , a method which is overwritten in the child class. Now, calling super.init() results in the overriden method being called, which in turn needs to use the name property, which is not initialized a this point.

The requirement to have all subclass storage initialized before calling super.init is to ensure object validity before any possible actions are called on it.