对象的内存布局：

• 第-个8个字节存储类相关的信息表的地址，
• 第二个8个存储引用计数相关的信息表的地址
• 后面开始存储实例属性的内存大小

Swift中多态的实现方式

• 底层实现与C++中的多态一致，采用的是虚表

• 根据在类信息地址找到虚表的首地址，虚表中存了类信息和函数的入口地址

  class Animal {
      var age = 12
      var num = 23
      
      func sepak() {
          print("Animal-Speak")
      }
      
      func eat() {
          print("Animal-eat")
      }
  }
  
  class Dog: Animal {
      var weight: Int = 0
      
      override func sepak() {
          print("Dog-sepak")
      }
      
      override func eat() {
          print("Dog-eat")
      }
  }
  
  class ErHa: Dog {
      var iq: Int = 0
      
  }
  
  // 0x600001257bc0
  /**
   A8 91 FB 01 01 00 00 00
   03 00 00 00 00 00 00 00
   0F 00 00 00 00 00 00 00
   17 00 00 00 00 00 00 00
   */
  let ani = Animal()
  ani.age = 15
  ani.sepak()
  
  // 0x600001285800
  /*
   88 92 FB 01 01 00 00 00
   03 00 00 00 00 00 00 00
   0A 00 00 00 00 00 00 00
   17 00 00 00 00 00 00 00
   14 00 00 00 00 00 00 00
   */
  let dog = Dog()
  dog.age = 10
  dog.weight = 20
  
  // 0x600001285950
  /**
   88 F3 AD 0E 01 00 00 00
   03 00 00 00 00 00 00 00
   0A 00 00 00 00 00 00 00
   17 00 00 00 00 00 00 00
   14 00 00 00 00 00 00 00
   1E 00 00 00 00 00 00 00
   */
  let erHa = ErHa()
  erHa.age = 10
  erHa.weight = 20
  erHa.iq = 30