block7.go

package main

import "io"
import "bytes"
import "fmt"

func main() {
    var w io.Writer
    fmt.Println(w == nil)   // True

    var b *bytes.Buffer
    w = b
    fmt.Println(w == nil)   // false

    write(w)                // panic: runtime error: invalid memory address or nil pointer dereference
}

// If out is non-nil, output will be written to it.
//
func write(out io.Writer) {
    // ...do something...
    if out != nil {                     // This guard is not secure enough
        out.Write([]byte("done!\n"))
    }
}