[Swift] Class Initialization Safety Checks


class Human {
    var name: String
    var age: Int
    
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

class Student: Human {
    var school: String
    
    init(name: String, age: Int, school: String) {
        self.school = school
        super.init(name: name, age: age)
    }
}

class UnivStudent: Student {
    var major: String
    
    init(name: String, age: Int, school: String, major: String) {
        self.major = major
        super.init(name: name, age: age, school: school)
    }
    
    convenience init(age: Int) {
        self.init(name: "unknown", age: age, school: "Stanford", major: "computer")
    }
}

Inheritance chain

Let's look at the safety checks the compiler needs
before a class's initialization process.

Safety checks

This is a process the compiler performs.
It runs 4 checks.
① All properties of the current class must be initialized before the parent class's init runs.

init(name: String, age: Int, school: String, major: String) {
    self.major = major
    super.init(name: name, age: age, school: school)
}

In the UnivStudent class, the current class's property is major.
Through self.major = major, it checks that all of UnivStudent's properties are initialized.
The parent class can't initialize properties that only the child class has, so this check is performed.

② Properties inherited from the parent must be assigned after the parent's initialization finishes.

init(name: String, age: Int, school: String, major: String) {
    self.major = major
    self.age = 14 // ERROR!
    super.init(name: name, age: age, school: school)
}

self.age is a property inherited from the Human class.
But it tried to initialize this value before calling the parent's initializer, so an error occurs.

③ Same as ②, in a convenience initializer you also have to initialize properties after calling the designated initializer.

convenience init(age: Int) {
    self.age = 14 // ERROR
    self.init(name: "unknown", age: age, school: "Stanford", major: "computer")
}

④ Before phase 1 of initialization is done, you can't call instance properties/methods' properties,
and you can't reference the self property.

class UnivStudent: Student {
    var major: String
    var me: UnivStudent = self // ERROR!
    var me: UnivStudent = { self }() // ERROR!
    var me: UnivStudent { self } // Okay
    
    init(name: String, age: Int, school: String, major: String) {
        self.major = major
        me = self // ERROR!
        super.init(name: name, age: age, school: school)
    }
    
    convenience init(age: Int) {
        self.init(name: "unknown", age: age, school: "Stanford", major: "computer")
    }
}

A stored property can't reference self while phase 1 initialization isn't complete.
But a computed property doesn't reference self, it just returns a value, so it's allowed.

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