good good study, day day up!

vendor/gopl.io/ch7/bytecounter/main.go

@@ -0,0 +1,35 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 173.
+
+// Bytecounter demonstrates an implementation of io.Writer that counts bytes.
+package main
+
+import (
+	"fmt"
+)
+
+//!+bytecounter
+
+type ByteCounter int
+
+func (c *ByteCounter) Write(p []byte) (int, error) {
+	*c += ByteCounter(len(p)) // convert int to ByteCounter
+	return len(p), nil
+}
+
+//!-bytecounter
+
+func main() {
+	//!+main
+	var c ByteCounter
+	c.Write([]byte("hello"))
+	fmt.Println(c) // "5", = len("hello")
+
+	c = 0 // reset the counter
+	var name = "Dolly"
+	fmt.Fprintf(&c, "hello, %s", name)
+	fmt.Println(c) // "12", = len("hello, Dolly")
+	//!-main
+}