你真的清楚GO语言的defer吗？

Defer

Go's defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns. It's an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return. The canonical examples are unlocking a mutex or closing a file.

官方文档

执行顺序

LIFO 依次压栈，执行顺序与defer的顺序相反

参数求值时机

The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes.Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions.

defer的函数的参数，在defer执行时就已经确定，而不是函数调用时：

type Test struct{
	a int
}
func (t Test) print(x int){
	fmt.Printf("a is %v, x is %v ", t.a, x)
}
func deferRun() int {
	test := Test{1}
	num := 1 
	defer test.print(num)
	num++
	test.a++
	
	return 0
}
func main(){
	deferRun()	
}

运行结果：

a is 1, x is 1

return时具体做了什么？

我们知道，defer 函数在return时调用，但是具体做了什么？

只需弄清楚三步：

1. 设置result
2. 执行defer
3. 返回result

来看具体例子：

func main() {
   res := deferRun()
   fmt.Println(res)//输出2
}
func deferRun() (res int) {
  num := 1
  defer func() {
    res++
  }()  
  return num
  // res = num 
  // res ++
  // return res
}

无论怎样的过程，从这三步分析就不会出错