Swift中类可以使用计算属性，即使用get和set来间接获取/改变其他属性的值

class student: NSObject {
    var age = 1;
    var num = 2;
    var sum :Int{
        get{
            return age+num
            
        }
        set(op){
            num = op-age
        }
    }   
}

        let astudent = student();
        print("\(astudent.sum)");
        astudent.sum=10;
        print("\(astudent.num)");