2018-11-07 22:24
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I have a simple piece of code where I want to convert elements of a slice into json and then print them out to my http.responseWriter.

for _, element := range customers {
    result, _ := json.Marshal(element)
    fmt.Fprintf(w, string(result))

However when I compile this I get the error "result declared and not used". If I add a simple line:

_ = result

Then everything compiles and works fine. Why does the compiler complain about this usage, and what is the correct way to do this in go?

Any insight is appreciated, my searches so far seem to indicate the call to Fprintf should count as a usage.

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我有一段简单的代码,我想在其中将切片的元素转换为json,然后将它们打印出来 我的http.responseWriter。

结果,_:= json.Marshal(element)
 fmt.Fprintf(w,字符串 (结果))

但是,当我对此进行编译时,出现错误“已声明结果且未使用结果”。 如果我添加一条简单的行:

  _ =结果

然后,所有内容均可编译并正常运行。 为何编译器会抱怨这种用法?执行此操作的正确方法是什么?


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1条回答 默认 最新

  • dongliehuan3925 2018-11-08 07:42

    The code in question does not result in the error posted, for proof, check it on the Go Playground.

    This error usually is (and the op confirmed it is too in this case) caused by having a local variable with same name outside of the block, and when using the short variable declaration, that shadows that variable.

    This error can be reproduced with the following code:

    var result []byte
    customers := []int{}
    w := os.Stdout
    for _, element := range customers {
        result, _ := json.Marshal(element)
        fmt.Fprintf(w, string(result))

    Attempting to compile and run it, we get the error (try it on the Go Playground):

    prog.go:10:6: result declared and not used

    Solution is to use a simple assignment instead of the short variable declaration if intention is to use the existing variable (in which case no new variable will be created), or use a different name for the variable if intention is not to use the outer, existing variable (but then the outer variable is to be removed or be used of course).

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