为什么运行时错误io.WriterString？ [英] why runtime error on io.WriterString?

问题描述

运行以下代码时，出现panic：运行时错误：无效的内存地址或零指针取消引用。我不明白为什么，并且似乎无法从io.WriteString（w，s）中发现我认为问题存在的错误。任何人都可以指引我正确的方向吗？

 包主
 $ b导入（
io
fmt 
 
 $ b $ func main（）{
s：=hei
 var w io.Writer 
 _，err：= io.WriteString（w ，s）
 if err！= nil {
 fmt.Println（s）
} 
} 
  

解决方案

如果您添加

  fmt .Println（w）
  

紧随 var w io.Writer ，你会看到打印的内容是< nil> 。这意味着你只是创建一个变量，而不是将其初始化为一个实际值。然后你试图将它传递给一个需要一个真正的 io.Writer 对象的函数，但是会得到一个零。

此外， io.Writer 是一个界面（请参阅 http ：//golang.org/pkg/io/#Writer ），所以你需要找到它的具体实现（比如 os.Stdout ）到能够实例化它。

有关 io 包的更多信息，请参阅 http://golang.org/pkg/io/ 。

PS 也许你会将此与C ++混淆;在C ++中，当您执行 io :: Writer w 时， w 会自动初始化为包含 io :: Writer ，然而，Go代码 var w io.Writer 实际上相当于 io :: Writer * w 在C ++中，很显然在这种情况下 w 将包含 null 或更可能是一些不确定的垃圾。 （Go确保它是 null ）。

I get a "panic: runtime error: invalid memory address or nil pointer dereference" when running the following code. I do not understand why and cant seem to catch the error from the io.WriteString(w, s) where I believe the problem resides. Can anybody point me in the right direction?

package main

import(
    "io"
    "fmt"
)

func main() {
    s := "hei"
    var w io.Writer
    _, err := io.WriteString(w, s)
    if err != nil{
    fmt.Println(s)
    }   
}

解决方案

If you add

fmt.Println(w)

right after var w io.Writer, you'll see that what gets printed is <nil>. This means you're just creating a variable but not initializing it to a real value. You then attempt to pass it to a function that needs a real io.Writer object but gets a nil.

Also, io.Writer is an interface (see http://golang.org/pkg/io/#Writer), so you need to find a concrete implementation of it (such as os.Stdout) to be able to instantiate it.

For more information about the io package, see http://golang.org/pkg/io/.

P.S. perhaps you're confusing this with C++; in C++, when you do io::Writer w, then w automatically gets initialized to contain a fresh copy of io::Writer, however, the Go code var w io.Writer is really equivalent to io::Writer* w in C++, and it's obvious that w in that case will contain null or more probably some indeterministic garbage. (Go guarantees that it's null though).

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