英文:
why runtime error on io.WriterString?
问题
当运行以下代码时，我遇到了一个"panic: runtime error: invalid memory address or nil pointer dereference"错误。我不明白为什么会出现这个错误，也无法捕捉到我认为问题出在io.WriteString(w, s)的错误。有人能指点我正确的方向吗？
package main
import (
"io"
"fmt"
)
func main() {
s := "hei"
var w io.Writer
_, err := io.WriteString(w, s)
if err != nil {
fmt.Println(s)
}
}
英文:
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)
}
}
答案1
得分: 6
如果你在 var w io.Writer 之后添加 fmt.Println(w)，你会发现打印出来的是 <nil>。这意味着你只是创建了一个变量，但没有将其初始化为一个真实的值。然后你尝试将它传递给一个需要真实的 io.Writer 对象的函数，但实际上传递的是一个空值。
另外，io.Writer 是一个接口（参见 http://golang.org/pkg/io/#Writer），所以你需要找到一个具体的实现（比如 os.Stdout）来实例化它。
有关 io 包的更多信息，请参见 http://golang.org/pkg/io/。
P.S. 也许你把它和 C++ 混淆了；在 C++ 中，当你写 io::Writer w 时，w 会自动初始化为一个包含 io::Writer 的新副本，然而，Go 代码中的 var w io.Writer 实际上等同于 C++ 中的 io::Writer* w，很明显，在这种情况下，w 将包含 null 或更可能是一些不确定的垃圾值。（不过 Go 保证它是 null）。
英文:
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).
答案2
得分: 2
var w io.Writer
将w初始化为nil写入器。你需要将它指向一个实际的写入器才能对其进行有用的操作，例如：
w = os.Stdout
英文:
var w io.Writer
initializes w to a nil writer. You need to point it to an actual writer to do anything useful with it, e.g.
w = os.Stdout
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