doumen1883
2014-04-17 16:32
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如何从Go中的匿名函数访问全局变量?

I have a code:

var test string

func main() {
    http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
        test = "index"
    })

    fmt.Println(test)

    if error := http.ListenAndServe(":9001", nil); error != nil {
        log.Fatal("Error!", error)
    }
}

How to change the value of the test variable in the anonymous function? I will be thankful!

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我有一个代码:

  var测试字符串
  
func main(){
 http.HandleFunc(“ /”,func(writer http.ResponseWriter,request * http.Request){
 test =“ index” 
})
 
 fmt.Println(test  )
 
如果错误:= http.ListenAndServe(“:9001”,nil);  error!= nil {
 log.Fatal(“ Error!”,error)
} 
} 
   
 
 

如何更改 test 变量中的匿名函数? 我会很感激!

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1条回答 默认 最新

  • douxiuyi6529 2014-04-17 16:49
    最佳回答

    The HTTP handler does change your global variable; however you don't protect access to the global variable and thus have a race condition. That is to say fmt.Println(test) runs before your http.HandleFunc.

    I'm assuming this is a toy example: If you want to change your code to wait for the value to be changed / wait for an HTTP hit, before terminating, then you could do this:

    var test string
    var doneCh = make(chan bool, 1)
    
    func main() {
        http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
            test = "index"
            select {
            case doneCh <- true:
            default:
            }
        })
    
        go func() {
            <-doneCh
            fmt.Println(test)
        }()
    
        if error := http.ListenAndServe(":9001", nil); error != nil {
            log.Fatal("Error!", error)
        }
    }
    

    This uses a channel to guard the state of test. In real code it would be more likely to use sync.Once or sync.Mutex. Also, I should mention (and I hope you already realize) mutating global state is always something to be avoided when possible.

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