什么是 Defer

延迟执行 Defer 语句用于在当前的 Defer 语句返回之前执行一个函数调用。文字不好理解，直接上代码，更容易理解一点。

package main

import "fmt"

func main() {
    sayHi("Boyce")
}

func sayHi(name string) {
defer sayBye()
    fmt.Println("Hi", name)
    fmt.Println("bla bla bla ...")
}

func sayBye() {
    fmt.Println("bye bye . . .")
}

// go run .deferSayHi.go
Hi  Boyce
bla bla bla ...
bye bye . . .

Defer 方法

Defer 不仅能用于上面的 function ，也可以用于下面的 method

package main

import "fmt"

type msg struct {
	sender  string
	subject string
	content string
}

func (m msg) msgInfo() {
	fmt.Println("subject:", m.subject)
	fmt.Println("from:", m.sender)
	fmt.Println("content is :", m.content)
}

func main() {
	m := msg{
		sender:  "Boyce",
		subject: "error log",
		content: "your site is down",
	}
defer m.msgInfo()
	fmt.Println("ding ding ding, you have a new alert ...")
}


// go run .deferMethod.go
ding ding ding, you have a new alert ...
subject: error log
from: Boyce
content is : your site is down

Defer 参数的计算

延迟函数的参数是在执行 defer 语句时求值，而不是在实际函数调用完成时求值。

用代码再解释一下：

package main

import "fmt"

func printDefer(input int) {
    fmt.Println("the value in defer is", input)
}

func main() {
    a := 2
defer printDefer(a)
    a = 10
    fmt.Println("the value before defer is", a)
}

// go run .deferParamCal.go
the value before defer is 10
the value in defer is 2

多 Defer 的执行顺序

package main

import "fmt"

func main() {
    sayHi("Boyce")
}

func sayHi(name string) {
for _, v := range []rune(name) {
defer fmt.Printf("%c", v)
    }
defer fmt.Println("I think your reversed name is ...")
    fmt.Println("Hi", name)
}

defer fmt.Printf("%c", e)
defer fmt.Printf("%c", c)
defer fmt.Printf("%c", y)
defer fmt.Printf("%c", o)
defer fmt.Printf("%c", B)
defer fmt.Println("I think your reversed name is ...")

//go run .deferStack.go
Hi Boyce
I think your reversed name is ...
ecyoB

总结

特别声明：以上文章内容仅代表作者本人观点，不代表变化吧观点或立场。如有关于作品内容、版权或其它问题请于作品发表后的30日内与变化吧联系。