go类型断言类型转换

package main
import "fmt"
func main() {
  var i interface{} = "hello"
  s := i.(string)
  fmt.Println(s)
  s, ok := i.(string)
  fmt.Println(s, ok)
  f, ok := i.(float64)
  fmt.Println(f, ok)
  f = i.(float64) // panic
  fmt.Println(f)
}

package main
import "fmt"
func do(i interface{}) {
  switch v := i.(type) {
  case int:
    fmt.Printf("Twice %v is %v\n", v, v*2)
  case string:
    fmt.Printf("%q is %v bytes long\n", v, len(v))
  default:
    fmt.Printf("I don't know about type %T!\n", v)
  }
}
func main() {
  do(21)
  do("hello")
  do(true)
}