error处理

class ABC {
      enum Error: ErrorTpe {
        case a
        case b 
    }
    
    func a(a: String?)  throws -> Int {
        //抛出异常后方法会立即终止，defer是成功执行或者失败后都会执行的block
        defer{
            print("have a nice day")
        }
        guard let a = a else {
            throw Error.a
        }
    }
}

let abc = ABC()
let string: String? = nil
//成功便执行 否者返回nil
try? abc.a(string)
 //do catch
do{
    try abc.a(string)    
}catch let error as ABC.Error{
      print(error)
}