Golang关键字defer的用法详解

func CopyFile(dstFile, srcFile string) (wr int64, err error) { 
    src, err := os.Open(srcFile)     
    if err != nil {         
        return     
    }      
    dst, err := os.Create(dstFile)     
    if err != nil {         
        return    
    }      
    wr, err = io.Copy(dst, src)     
    dst.Close()     
    src.Close()     
    return 
}

func CopyFile(dstFile, srcFile string) (wr int64, err error) {
    src, err := os.Open(srcFile)
    if err != nil {
        return
    }
    defer src.Close()
    dst, err := os.Create(dstFile)
    if err != nil {
        return
    }
    defer dst.Close()
    wr, err = io.Copy(dst, src)  
    return err
}

package main
import "fmt"
func main() {
   defer func() {
      if r := recover(); r != nil {
         fmt.Println(r)
      }
   }()
   a := 1
   b := 0
   fmt.Println("result:", a/b)   
}

func deferRun1() {
   var num = 1
   numptr := &num
   defer fmt.Println("defer run 1: \n", numptr, num, *numptr) // 0xc000022088 1 1
   defer func() {
      fmt.Println("defer run 2 : \n", numptr, num, *numptr) // 0xc000116028 2 2
   }()
   defer func(num int, numptr *int) {
      fmt.Println("defer run 3: \n", numptr, num, *numptr) //0xc000116028 1 2
   }(num, numptr)
   num = 2
   fmt.Println(numptr, num, *numptr) // 0xc000116028 2 2
   return
}

func main() {
   deferRun()
   deferRun2()
}
func deferRun() {
   var num = 1
   defer fmt.Printf("num is %d\n", num)
   num = 2
   return
}
func deferRun2() {
   var arr = [4]int{1, 2, 3, 4}
   defer printArr(&arr)
   arr[0] = 100
   return
}
func printArr(arr *[4]int) {
   for i := range arr {
      fmt.Println(arr[i])
   }
}

func main() {
   res1 := deferRun()
   fmt.Println(res1)
   res2 := deferRun2()
   fmt.Println(res2)
   res3 := deferRun3()
   fmt.Println(res3)
   res4 := deferRun4()
   fmt.Println(res4)
}
// return 2 
func deferRun() (res int) {
   num := 1
   defer func() {
      res++
   }()
   return num   //1.返回值参数res赋值为num即1  //2.执行defer语句，res++  //3. 结果返回2
}
// return 1
func deferRun2() int {   // 注意这里返回值是匿名的，我们可以在心里为这个匿名的返回值取名为res
   var num int
   defer func() {
      num++
   }()
   return 1  //1.匿名返回值参数(res)赋值为1  //2.执行defer语句，num++,加了个寂寞 //3.结果返回1
}
//return 1
func deferRun3() int {
   num := 1
   defer func() {
      num++
   }()
   return num  //1.匿名(res)赋值为num即1  //2.执行defer语句，num++,加了个寂寞，num为2，res依旧为1 
              //3.结果返回1
}
//return 2
func deferRun4() (res int) {
   num := 1
   defer func() {
      res++
   }()
   return num //1.返回值参数res赋值为1  //2.执行defer语句，res++ //3.结果返回2
}