io.Writeインタフェースを持つ構造体Hogeの例

interface_ex1.go

package main

import (
  "fmt"
)

type Hoge struct {
  label string
  cnt   int
}

func (h *Hoge) Write(b []byte) (n int, err error) {
  h.cnt++
  fmt.Printf("(%s:%d) len=%d bytes\n", h.label, h.cnt, len(b))
  return len(b), nil
}

func main() {
  h := &Hoge{"hoge1", 0}
  fmt.Printf("%#v\n", h)

  fmt.Fprintf(h, "Hello, world")
  fmt.Fprintf(h, "Hello, world once again")

  fmt.Printf("%#v\n", h)
}

実行結果 - The Go Playground

&main.Hoge{label:"hoge1", cnt:0}
(hoge1:1) len=12 bytes
(hoge1:2) len=23 bytes
&main.Hoge{label:"hoge1", cnt:2}