go - defer 语句是如何工作的

Question

我尝试学习 golang 并使用有效的 go 作为讲座。我卡在capital defer上了，看下面的代码

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()
}

我得到的输出

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

我知道，defer 语句将在函数中的 return 语句之后执行。但是在这里，为什么输入:b是第一个输出呢？我预计在 b 中作为第一输出!

Answer 1

根据 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.

所以延迟函数的参数(在你的例子中是trace("b"))先求值