casting - Go:将 map[string]interface{} 转换为 map[string]string 的类型失败

Question

我不确定为什么以下转换不起作用:

import "fmt"

func main() {
    v := map[string]interface{}{"hello": "world"}
    checkCast(v)

}

func checkCast(v interface{}) {
    _, isCorrectType := v.(map[string]string)
    if !isCorrectType { 
        fmt.Printf("incorrect type")  <------------- why does it enter this if statement?
        return
    }
}

Answer 1

---

map[string]interface{} 与 map[string]string 不同。 interface{} 类型与 string 类型不同。

如果它们都是map[string]string:

package main

import "fmt"

func main() {
    v := map[string]string{"hello": "world"}
    checkCast(v)

}

func checkCast(v interface{}) {
    _, isCorrectType := v.(map[string]string)
    if !isCorrectType {
        fmt.Printf("incorrect type")
        return
    }
}

输出:

[no output]

---

语句 v.(map[string]string) 是一个类型断言，而不是强制转换。

The Go Programming Language Specification

Type assertions

For an expression x of interface type and a type T, the primary expression

x.(T)

asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

---

Go 有转化。

The Go Programming Language Specification

Conversions

Conversions are expressions of the form T(x) where T is a type and x is an expression that can be converted to type T.