函数自动执行而不被调用

Question

I started to learn Go yesterday on the Tour of Go website and I thought that I was understanding everything well;

But then I was totally confused on the exercise Stringers: https://tour.golang.org/methods/18

So I searched a solution to see what I was doing wrong and I found this:

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.

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

 }

func main() {
    hosts := map[string]IPAddr{
        "loopback":  {127, 0, 0, 1},
        "googleDNS": {8, 8, 8, 8},
    }
    for name, ip := range hosts {
        fmt.Printf("%v: %v
", name, ip)
    }
}

and the output is this:

loopback: 127.0.0.1
googleDNS: 8.8.8.8

it left me completely confused;

Why did the function activate itself? Should'nt it be like: i.String() inside the main func?

Why did the fmt.Printf show nothing on the screen at all?

Or the fmt.Printf returned that because the String() changed the value?

I'm sorry if the question is confusing, but can someone explain this to me?

Answer 1

If you look at the previous page in the tutorial, you see that Stringer is an interface defined in the fmt package. Implementing the String() method automatically makes IPAddr implement this interface.

The fmt.Printf function checks if its argument implements the Stringer interface. Since it now does, it calls the String method when it needs to figure out how to show an IPAddress (you can change this behavior by using a format specifier other than %v, but that's beyond the scope of the question).

The fmt.Printf call showed something; it showed the lines

loopback: 127.0.0.1
googleDNS: 8.8.8.8

It didn't change their value or anything; it just called the String method to figure out how to write down an IPAddr, and wrote it down that way.