2013-10-05 10:21


  • io

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


func main() {
    s := "hei"
    var w io.Writer
    _, err := io.WriteString(w, s)
    if err != nil{
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  • duankuaiwang2706 duankuaiwang2706 8年前

    If you add


    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, 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

    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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  • dongyingla8668 dongyingla8668 8年前
    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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