Defer

基本用法

• 参数会在 defer 方法执行时，固化

func a() {
    i := 0
    defer fmt.Println(i) // 0
    i++
    return
}

• 执行顺序后进先出

func b() {
    for i := 0; i < 4; i++ {
        defer fmt.Print(i)
    }
}
// 3210

• 操作返回值变量

func c() (i int) {
    defer func() { i++ }()
    return 1
}

调整错误返回值

func Create() (err error) {
    defer func() {
        if err != nil {
            db.Rollback()
        } else {
            err = db.Commit()
        }
    }()
}

示例：

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

defer 可嵌套使用

package main
import (
    "fmt"
)
func main() {
    defer func() {
        defer func() {
            fmt.Println("Inner defer")
        }()
        fmt.Println("Outer defer")
    }()
    fmt.Println("Execution finished")
}
// Execution finished
// Outer defer
// Inner defer

配合 panic

package main
import "fmt"
func main() {
    f()
    fmt.Println("Returned normally from f.")
}
func f() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered in f", r)
        }
    }()
    fmt.Println("Calling g.")
    g(0)
    fmt.Println("Returned normally from g.")
}
func g(i int) {
    if i > 3 {
        fmt.Println("Panicking!")
        panic(fmt.Sprintf("%v", i))
    }
    defer fmt.Println("Defer in g", i)
    fmt.Println("Printing in g", i)
    g(i + 1)
}
// Calling g.
// Printing in g 0
// Printing in g 1
// Printing in g 2
// Printing in g 3
// Panicking!
// Defer in g 3
// Defer in g 2
// Defer in g 1
// Defer in g 0
// Recovered in f 4
// Returned normally from f.