golang-101-hacks(21)——类型断言&类型开关

注：本文是对golang-101-hacks中文翻译
通过类型断言(type assertion)方式来判断接口的具体类型，
Sometimes, you may want to know the exact type of an interface variable. In this scenario, you can use type assertion:

x.(T)

x”的类型必须为interface的变量，“T”表示是推断的类型。例如:
x is the variable whose type must be interface, and T is the type which you want to check. For example:

package main

import "fmt"

func printValue(v interface{}) {
    fmt.Printf("The value of v is: %v", v.(int))
}

func main() {
    v := 10
    printValue(v)
}

运行结果如下

The value of v is: 10

在上面的例子中，使用' v.(int) '来断言' v '是整数(int)类型。
In the above example, using v.(int) to assert the v is int variable.
如果“类型断言”推断错误，将会导致程序运行恐慌(panic):将下面断言语句
if the type assertion operation fails, a running panic will occur: change

fmt.Printf("The value of v is: %v", v.(int))  

修改成如下

fmt.Printf("The value of v is: %v", v.(string))

程序运行结果会出现错误

panic: interface conversion: interface is int, not string

goroutine 1 [running]:
panic(0x4f0840, 0xc0820042c0)
......

为了提高程序的健壮性，类型断言type assertion实际上返回一个额外的布尔变量来判断这个断言类型是否正确。将程序修改成如下:

To avoid this, type assertion actually returns an additional boolean variable to tell whether this operations holds or not. So modify the program as follows:

package main

import "fmt"

func printValue(v interface{}) {
    if v, ok := v.(string); ok {
        fmt.Printf("The value of v is: %v", v)
    } else {
        fmt.Println("Oops, it is not a string!")
    }

}

func main() {
    v := 10
    printValue(v)
}

这次运行结果如下

Oops, it is not a string!

此外，您还可以使用“type switch”，通过“type assertion”来确定变量的类型，并执行相应的操作。如下面的例子:
Furthermore, you can also use type switch which makes use of type assertion to determine the type of variable, and do the operations accordingly. Check the following example:

package main

import "fmt"

func printValue(v interface{}) {
    switch v := v.(type) {
    case string:
        fmt.Printf("%v is a string\n", v)
    case int:
        fmt.Printf("%v is an int\n", v)
    default:
        fmt.Printf("The type of v is unknown\n")
    }
}

func main() {
    v := 10
    printValue(v)
}

运行结果如下

10 is an int

与类型断言不同，“type switch”在括号中并没有指定的变量类型(例如“int”)，而使用了关键字type。
参考:
Effective Go;
Go – x.(T) Type Assertions;
How to find a type of a object in Golang?.