Defer in Go

很多现代的编程语言中都有 defer 关键字，Go 语言的 defer 会在当前函数或者方法返回之前执行传入的函数。它会经常被用于关闭文件描述符、关闭数据库连接以及解锁资源。

When it comes to keyword defer in go, I felt confused.

The teacher showed a piece of code:

func main() {
  i := 1
  defer fmt.Println(i + 1)

  i++
  fmt.Println("hello, world!")
}

I thought that it must print "hello, world!" and 3 at first. But after typing and runing, I was totally shocked —— It’s output is "hello, world" and 2!

What actually gets printed as a two, because that i +1 is evaluated right there when the first statement, when it hits the differed statement, the i +1 is evaluated and at that time, i is a one, so i +1 is a two. So, later when the differed statement actually gets executed it’s still prints a two.