为什么io.WriterString出现运行时错误？

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)
    }   
}

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).

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