类 class 是引用类型
1.class 类的类型 { var 类的属性：类型...}
class Person {
var firstName: String
var lastName: String
// 初始化
init (firstName:String , lastName:String){
self.firstName = firstName
self.lastName = lastName
}
}
2.类的初始化
var Person1 = Person(firstName:"jack" , lastName:"son")
3.类的使用
Person1.firstName