斯金格斯的困惑行为？

Question

New to Go, so please bear with me.

I've been looking at the "Tour of Go" pages, and stumbled into something puzzling about Stringers. Consider the exercise at https://tour.golang.org/methods/18

My initial answer was implementing

func (this *IPAddr) String() string {
  return fmt.Sprintf("%d.%d.%d.%d", this[0], this[1], this[2], this[3])
}

however, this is not used f main prints just fmt.Printf("%v: %v ", name, ip). If I change the print to fmt.Printf("%v: %v ", name, ip.String()), then it is used whether the receiver type is *IPAddr or IPAddr).

why is this happening?

Answer 1

Because you're passing an IPAddr value to fmt.Printf, your String() method isn't part of the method set. Your solution works if you pass in a pointer:

fmt.Printf("%v: %v
", name, &ip)

But a general solution is to not use a pointer receiver:

func (ip IPAddr) String() string {
  return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
}

This way the String() method can be used from an IPAddr, which is what you're passing to Printf, or an *IPAddr, which includes the methods of the value receiver.