推迟使用说明

Question

Let's assume I have the following function

func printNumbers(){
 var x int

 defer fmt.Println(x)

 for i := 0; i < 5; i++{
  x++
 }
}

As it is said in the specification:

Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked.

Obviously, zero will be printed out when the function execution ends. But what should I do if I want to print out the final value of variable x?

I've come up with the following solution:

func printNumbers(){
  var x int

  printVal := func(){
    fmt.Println(x)
  }

  defer printVal()

  for i := 0; i < 5; i++{
    x++
  }
}

So I wonder if there is a better way to resolve this problem.

Answer 1

If the defer has arguments they are evaluated at the line of the defer-statement; this is illustrated in the following snippet, where the defer will print 0:

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

You can use an anonymous function as a defer statement if you want to postpone the execution of a statement or a function until the end of the enclosing (calling) function. Here is an updated example:

func printNumbers() {
    x := 0
    defer func() { fmt.Println(x) }()
    for i:=0; i < 5; i++ {
        x++;
    }
    return
}

http://play.golang.org/p/YQGQ_8a0_9