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本文次要钻研一下golang的类型断言

类型断言

x.(T)
  • 断言x不为nil且x为T类型
  • 如果T不是接口类型，则该断言x为T类型
  • 如果T类接口类型，则该断言x实现了T接口

实例1

func main() {    var x interface{} = 7    i := x.(int)    fmt.Println(reflect.TypeOf(i))    j := x.(int32)    fmt.Println(j)}
间接赋值的形式，如果断言为true则返回该类型的值，如果断言为false则产生runtime panic；j这里赋值间接panic

输入

intpanic: interface conversion: interface {} is int, not int32goroutine 1 [running]:main.main()        type_assertion.go:12 +0xdaexit status 2

不过个别为了防止panic，通过应用ok的形式

func main() {    var x interface{} = 7    j, ok := x.(int32)    if ok {        fmt.Println(reflect.TypeOf(j))    } else {        fmt.Println("x not type of int32")    }}

switch type

另外一种就是variable.(type)配合switch进行类型判断

func main() {    switch v := x.(type) {    case int:        fmt.Println("x is type of int", v)    default:        fmt.Printf("Unknown type %T!\n", v)    }}

判断struct是否实现某个接口

type shape interface {    getNumSides() int    getArea() int}type rectangle struct {    x int    y int}func (r *rectangle) getNumSides() int {    return 4}func (r rectangle) getArea() int {    return r.x * r.y}func main() {    // compile time Verify that *rectangle implement shape    var _ shape = &rectangle{}    // compile time Verify that *rectangle implement shape    var _ shape = (*rectangle)(nil)    // compile time Verify that rectangle implement shape    var _ shape = rectangle{}}

输入

cannot use rectangle literal (type rectangle) as type shape in assignment:        rectangle does not implement shape (getNumSides method has pointer receiver)

小结

  • x.(T)能够在运行时判断x是否为T类型，如果间接应用赋值，当不是T类型时则会产生runtime panic
  • 应用var _ someInterface = someStruct{}能够在编译期间校验某个struct或者其指针类型是否实现了某个接口

doc

  • Type_assertions
  • Go : Check if type implements an interface