golang之defer

 1 // Contents returns the file‘s contents as a string.
 2 func Contents(filename string) (string, error) {
 3     f, err := os.Open(filename)
 4     if err != nil {
 5         return "", err
 6     }
 7     defer f.Close()  // f.Close will run when we‘re finished.
 8 
 9     var result []byte
10     buf := make([]byte, 100)
11     for {
12         n, err := f.Read(buf[0:])
13         result = append(result, buf[0:n]...) // append is discussed later.
14         if err != nil {
15             if err == io.EOF {
16                 break
17             }
18             return "", err  // f will be closed if we return here.
19         }
20     }
21     return string(result), nil // f will be closed if we return here.
22 }

func trace(s string) string {
    fmt.Println("entering:", s)
    return s
}

func un(s string) {
    fmt.Println("leaving:", s)
}

func a() {
    defer un(trace("a"))
    fmt.Println("in a")
}

func b() {
    defer un(trace("b"))
    fmt.Println("in b")
    a()
}

func main() {
    b()
}

entering: b
in b
entering: a
in a
leaving: a
leaving: b

1 for i := 0; i < 5; i++ {
2     defer fmt.Printf("%d ", i)
3 }