聊聊golang的类型断言

序

本文主要研究一下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)
}

直接赋值的方式，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这里赋值直接panic

输出

int
panic: interface conversion: interface {} is int, not int32

goroutine 1 [running]:
main.main()
        type_assertion.go:12 +0xda
exit status 2

不过一般为了避免panic，通过使用ok的方式

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{}
}

输出

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)
var _ someInterface = someStruct{}

doc

• Type_assertions
• Go : Check if type implements an interface

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