golang中的类型断言只能对于接口类型变量，其它类型变量的类型是确定的，否则编译器会报错

    1. v := varI.(T)

    标准类型断言：

    1. if v, ok := varI.(T); ok {
    2. //dosoming(v)
    3. }
    4. //varI is not of type T
    6. if _, ok := varI.(T); ok {
    7. //todo
    8. }

    example:

    1. package main
    3. import (
    4.     "fmt"
    5.     "math"
    6. )
    8. type Square struct {
    9.     side float32
    10. }
    12. type Circle struct {
    13.     radius float32
    14. }
    16. type Shaper interface {
    17.     Area() float32
    18. }
    20. func main() {
    21.     var areaIntf Shaper
    22.     sq1 := new(Square)
    23.     sq1.side = 5
    25.     areaIntf = sq1
    26.     // Is Square the type of areaIntf?
    27.     if t, ok := areaIntf.(*Square); ok {
    28.         fmt.Printf("The type of areaIntf is: %T\n", t)
    29.     }
    30.     if u, ok := areaIntf.(*Circle); ok {
    31.         fmt.Printf("The type of areaIntf is: %T\n", u)
    32.     } else {
    33.         fmt.Println("areaIntf does not contain a variable of type Circle")
    34.     }
    35. }
    37. func (sq *Square) Area() float32 {
    38.     return sq.side * sq.side
    39. }
    41. func (ci *Circle) Area() float32 {
    42.     return ci.radius * ci.radius * math.Pi
    43. }

    备注
    如果忽略 areaIntf.(*Square) 中的 * 号，会导致编译错误：impossible type assertion: Square does not implement Shaper (Area method has pointer receiver)。