了解defer

介绍

A “defer” statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a eturn statement, reached the end of its function body, or because the corresponding goroutine is panicking.

作用

• 将被defer指定的函数 延迟到 外层函数返回之前执行。
• 当相关联的协程发生了 panic后执行

语法

defer expression

func TestDefer(t *testing.T){
  defer func(){
    fmt.Println("defer run")
  }
  fmt.Println("TestDefer run")
}

其中expression必须为一个方法或者函数

defer结构体

type _defer struct {
   siz     int32 // includes both arguments and results
   started bool   //此defer是否已经执行了
   heap    bool
   // openDefer indicates that this _defer is for a frame with open-coded
   // defers. We have only one defer record for the entire frame (which may
   // currently have 0, 1, or more defers active).
   openDefer bool
   sp        uintptr  // sp at time of defer
   pc        uintptr  // pc at time of defer
   fn        *funcval // can be nil for open-coded defers  defer对应的函数指针
   _panic    *_panic  // panic that is running defer       defer对应的panic指针
   link      *_defer   //下一个需要执行的defer对应指针

   // If openDefer is true, the fields below record values about the stack
   // frame and associated function that has the open-coded defer(s). sp
   // above will be the sp for the frame, and pc will be address of the
   // deferreturn call in the function.
   fd   unsafe.Pointer // funcdata for the function associated with the frame
   varp uintptr        // value of varp for the stack frame
   // framepc is the current pc associated with the stack frame. Together,
   // with sp above (which is the sp associated with the stack frame),
   // framepc/sp can be used as pc/sp pair to continue a stack trace via
   // gentraceback().
   framepc uintptr
}

作用时机

if the surrounding function returns through an explicit return statement, deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller

如果外层函数有明确的返回值，那么在 返回值赋值给结果变量后，但在将结果变量返回给调用者前，defer函数此时执行。

func TestDefer() int{
    i := 1
    defer(){
        i++;
    }
    return i
}

调用这个函数最终返回的值是 1

注意：
return操作不是原子操作，而是分成两个步骤：

1. 将需要return的value赋值给一个变量
2. 将变量返回

上述示例函数的实际执行顺序：

anynomous := i
defer函数
return anynomous

anynomous是return过程中的一个匿名变量

defer函数的预计算

defer关键字调用时，会立刻拷贝函数中引用的外部参数；

func TestDeferCalcu(t *testing.T) {
   i:=1
   defer func(i int){
      fmt.Println(i)
   }(i)
   i++
}

输出：1

当调用 defer func(i int){xxx}(i)时，会立刻将所引用的参数i给赋值；因此defer语句之后的 i++对defer内部的函数无影响;

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• 本文作者： xczll
• 本文链接： https://xczllgit.github.io/2021/08/07/golang/2021-08-07-defer/
• 版权声明： 本博客所有文章除特别声明外，均采用 MIT 许可协议。转载请注明出处！