class Student {
    let name: String
    init(name: String) {
        self.name = name
    }
    var scoreCard: ScoreCard?{
        didSet{
            print("\(name) got \(scoreCard!.marks) marks in this exam")
        }
    }
}
class ScoreCard {
    let marks: Float
    init(marks: Float) {
        self.marks = marks
    }
    var student: Student?{
        didSet{
            print("\(marks) for \(student!.name) in this exam")
        }
    }
}
var student: Student?
var score:ScoreCard?
student = Student(name: "Albert Einstein")
score = ScoreCard(marks: 80)
student!.scoreCard = score
score!.student = student
student = nil
score = nil