Extension Class Can do

• add init function

```swift

enum MyBool: Int {

case myTrue, myFalse

}

extension MyBool: ExpressibleByBooleanLiteral {

init(booleanLiteral value: Bool) {

self = value ? .myTrue : .myFalse
}

}
```

• add instance/type methods

• add calculate properties

• add runtime properties

• add conforming protocol

Extension Class cannot do

• cannot add "required" init
  不能定义 required 的初始化⽅法的。
  我们⽆法为现有的⾮ final 的 class 添加required init

• cannot add stored properties