swift5.0 多态

//多态
class A
{
    var name:String
    
    init(name:String) {
        self.name = name
    }
    
    func printName()
    {
        print("this is A \(name)")
    }
}

class B : A
{
    override func printName() {
        print("this is B:\(name)")
    }
    
    func play()
    {
        print("this is B play")
    }
}

var s1:A = A(name: "a")
print(s1.name)

s1.printName()

var s2:B = B(name: "b")

s2.printName()
s2.play()