延期陈述书如何运作

Question

i try to learn golang and use effective go as lecture. I stuck on capital defer, look at the following code

package main

import "fmt"

func trace(s string) string {
    fmt.Println("entering:", s)
    return s
}

func un(s string) {
    fmt.Println("leaving:", s)
}

func a() {
    defer un(trace("a"))
    fmt.Println("in a")
}

func b() {
    defer un(trace("b"))
    fmt.Println("in b")
    a()
}

func main() {
    b()
}

as output i've got

entering: b
in b
entering: a
in a
leaving: a
leaving: b

I know, that defer statement will be execute after return statement in the function. But here, why entering: b is the first output? I am expected in b as the fist output!

Answer 1

According to http://golang.org/doc/effective_go.html#defer:

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.

So argument of defer function (trace("b") in your case) evaluates first