golang 錯誤處理

一、defer

package main

import (
	"fmt"
	"os"

	"bufio"
)

func tryDefer() {
	for i := 0; i < 100; i++ {
		defer fmt.Println(i)
		if i == 30 {
			// Uncomment panic to see
			// how it works with defer
			panic("printed too many")
		}
	}
}

func writeFile(filename string) {
	file, err := os.OpenFile(filename,
		os.O_EXCL|os.O_CREATE|os.O_WRONLY, 0666)

	if err != nil {
		if pathError, ok := err.(*os.PathError); !ok {//錯誤處理，向下轉型
			panic(err)
		} else {
			fmt.Printf("%s, %s, %s\n",
				pathError.Op,
				pathError.Path,
				pathError.Err)
		}
		return
	}
	defer file.Close()//無論return、panic最後都會被執行

	writer := bufio.NewWriter(file)
	defer writer.Flush()//先進後執行，後進先執行，退出時先執行Flush，再執行Close

	f := Fibonacci()
	for i := 0; i < 20; i++ {
		fmt.Fprintln(writer, f())
	}
}

func main() {
	tryDefer()
	writeFile("fib.txt")
}

輸出：

30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
panic: printed too many

goroutine 1 [running]:
main.tryDefer()
        /Users/jltxgcy/go/src/learn/errhandling/defer/defer.go:24 +0x111
main.main()
        /Users/jltxgcy/go/src/learn/errhandling/defer/defer.go:56 +0x20
defer執行的順序是先進後執行，無論是return還是panic，defer都會執行，用於關閉資源等，通常是成對出現。