swift类型转换（as操作符）

类型转换就是将一个类型转换为另一个类型，类型转换只能发生在子父类之间。

其实类型转换就是java里边的父类的引用指向了子类的对象，有多态的嫌疑，哈哈哈。

类型转换操作符有两种形式：as? 和 as! 。
as? 操作符将对象转换为目标类型的可选类型，转换失败返回nil
as! 操作符会强制解包，转换失败会触发运行时错误

class Person{
    var name:String
    init (name:String){
        self.name = name
    }
}

class Teacher:Person{
    var school:String
    init (name:String, school:String){
        self.school = school
        super.init(name: name)
    }
}

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

let tea = Teacher(name:"老师", school:"北工大")
let stu = Stutent(age:23, name:"学生")
let array = [tea, stu];
for item in array{
    if let temp=item as? Teacher{
        print("1",temp.school)
    }else if let temp=item as? Stutent{
        print("2",temp.age)
    }
}