defer里面的值何时evaluate

第一个问题

我在写这个的时候只是想记录一下像标题中说的, defer 里面的 parameters 何时 evaluate, 就像下面这一个例子. (后面才发现还有 function value 何时 evaluate 的问题)

例子来自于 https://groups.google.com/forum/#!topic/golang-nuts/c7YUK65Xqgs%5B1-25%5D

package main
import (
    "fmt"
)
func main() {
    for i := 0; i < 10; i++ {
        defer func() {
            fmt.Printf("func(): %v\n", i)
        }()
        defer printi(i)
        defer fmt.Printf("i: %v\t", i)
    }
}
func printi(i int) {
    fmt.Printf("printi(): %v\t", i)
}

结果是这样的

OUTPUT
======
i: 9	printi(): 9	func(): 10

i: 8	printi(): 8	func(): 10

i: 7	printi(): 7	func(): 10

i: 6	printi(): 6	func(): 10

i: 5	printi(): 5	func(): 10

i: 4	printi(): 4	func(): 10
i: 3	printi(): 3	func(): 10
i: 2	printi(): 2	func(): 10

i: 1	printi(): 1	func(): 10

i: 0	printi(): 0	func(): 10

二楼的 emepyc 说的很清楚, 我也不画蛇添足了, 直接复制过来

The key point is that the arguments to the deferred functions are evaluated and copied in the invocation of the defer. So:

defer  func()  {
  fmt.Printf("func():  %v\n",  i)
}()

No arguments to func(), so i is not copied and the final value of i is used when the deferred function is executed.

defer  printi(i)
defer  fmt.Printf("i:  %v\t",  i)

In both cases, i is the argument of the deferred function, so its value is copied and used when the deferred function is evaluated.

第二个问题

第二个问题, 在我看完之后, 并看了相关的链接资料, 发现是第一个问题的超集.

先来看一下原问题, https://stackoverflow.com/questions/51360229/the-deferred-calls-arguments-are-evaluated-immediately

这个回答里面也给出了 golang 官方资料关于 defer 的链接, 我之前在 google 搜索 golang defer 在第一页居然没有出现这个官方链接 .. https://golang.org/ref/spec#Defer_statements . 官方资料里面言简意赅的解释了 defer 的执行规则, 但是… 如果不能精确理解其中一些术语的话, 会看的很不明白, 比如我..

由三句话组成, 分别对应三个执行规则.(我是这么理解的, 不精准)

1. 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.

2. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, 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.

3. If a deferred function value evaluates to nil, execution panics when the function is invoked, not when the “defer” statement is executed.

对于第一点, https://stackoverflow.com/questions/51360229/the-deferred-calls-arguments-are-evaluated-immediately 这里有非常好的解释了.

第二点, 在 Defer, Panic, and Recover 中第三个例子有解释

func c() (i int) {
    defer func() { i++ }()
    return 1
}

像上面这段代码, 是返回2

在 https://golang.org/ref/spec#Defer_statements 也有例子

第三点是说, 如果 deferred function value 返回了nil, 不会马上出错, 而是等到外层函数返回之后调用到defer函数的时候才会panic, 如下:

package main

import "fmt"

func def(s string) func() {
	fmt.Println("tier up")
	fmt.Println(s)
	return nil
}

func main() {
	defer def("defered line")()
	fmt.Println("main")
}

执行结果

% go run a.go
tier up
defered line
main
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x10528e8]

goroutine 1 [running]:
main.main()
	/private/tmp/1577163316/a.go:14 +0xc8
exit status 2