1 func Printf(format string, a ...interface{}) (n int, err error){
 2     return Fprintf(os.Stdout, format, a...)
 3 }
 4 
 5 func Sprintf(format string,  args ...interface{}) string{
 6     var buf bytes.Buffer
 7     Fprintf(&buf, format, args...)
 8     return buf.String()
 9 }
10 
11 func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error){
12     ...
13 }

 1 package io
 2 // Writer is the interface that wraps the basic Write method.
 3 type Writer interface {
 4     // Write writes len(p) bytes from p to the underlying data stream.
 5     // It returns the number of bytes written from p (0 <= n <= len(p))
 6     // and any error encountered that caused the write to stop early.
 7     // Write must return a non-nil error if it returns n < len(p).
 8     // Write must not modify the slice data, even temporarily.
 9     //
10     // Implementations must not retain p.
11     Write(p []byte) (n int, err error)
12 }

 1 package main
 2 
 3 import "fmt"
 4 
 5 type Msg struct {
 6     Id   int 
 7     Name string
 8 }
 9 
10 func (this *Msg) Write(p []byte) (int, error) {
11     this.Id += int(len(p))
12     this.Name = "interface"
13     return this.Id, nil 
14 }
15 
16 func main() {
17     msg := new(Msg)
18     msg.Id = 100 
19     msg.Name = "interface test print"
20 
21     var name = "Dolly"
22     fmt.Fprintf(msg, "%s", name) //convert msg, call Write(my func)
23     fmt.Println(msg)             //stdout
24 }