英文:
Pass pointer to go defer function doesn't work
问题
在我的代码中，我尝试使用numAddr来记录在defer语句之后num的变化。
func deferRun() {
num := 1
numAddr := &num
defer fmt.Printf("num is %d", *numAddr)
num = 2
return
}
func main() {
deferRun()
}
但是我得到的结果是num is 1，而不是2，为什么defer函数使用的是*numAddr的值而不是地址？
然后我尝试了另一种方式
func deferRun() {
num := 1
numAddr := &num
defer func(intAddr *int){
fmt.Printf("num is %d", *numAddr)
}(numAddr)
num = 2
fmt.Println("num is", *numAddr)
return
}
func main() {
deferRun()
}
这次它起作用了，我得到了num is 2，所以我认为可能是defer fmt.Printf(something)在声明时立即存储了字符串，而当defer函数实际运行时并没有使用numAddr？
英文:
In my code I try to use numAddr to record the change of num after the defer statement
func deferRun() {
num := 1
numAddr := &num
defer fmt.Printf("num is %d", *numAddr)
num = 2
return
}
func main() {
deferRun()
}
but I get num is 1 instead of 2, why does the defer function use the value instead of address of *numAddr?
then I try another way
func deferRun() {
num := 1
numAddr := &num
defer func(intAddr *int){
fmt.Printf("num is %d", *numAddr)
}(numAddr)
num = 2
fmt.Println("num is", *numAddr)
return
}
func main() {
deferRun()
}
This time it works and I get num is 2, so I think maybe defer fmt.Printf(something) stored the string immediately when it was declared and the numAddr is not used when the defer function actually run?
答案1
得分: 2
有趣的问题。为了回答这个问题，你必须了解一个规则，就像在这个Go教程中所说的那样：https://go.dev/tour/flowcontrol/12
延迟调用的参数会立即求值，但函数调用直到周围的函数返回时才执行。
示例1： 告诉延迟函数打印位于指定内存地址的值。
func deferRun() {
num := 1
numAddr := &num // 变量num在堆栈内存中的地址，例如0xc000076f38
defer func(intAddr *int){
fmt.Printf("num is %d", *numAddr)
}(numAddr) // 嘿，Go，在周围的函数返回时，打印位于这个地址的值（numAddr=0xc000076f38）
num = 2 // 现在位于地址0xc000076f38的值是2
return
}
输出将是2。
示例2： 告诉延迟函数打印指定的值。
func deferRun() {
num := 1
numAddr := &num // 变量num在堆栈内存中的地址，例如0xc000076f38
defer fmt.Printf("num is %d", *numAddr) // 嘿，Go，在周围的函数返回时，打印这个值*numAddr（*numAddr现在是1）
num = 2 // 现在位于地址0xc000076f38的值是2，但你告诉延迟函数在之前打印1
return
}
输出将是1。
英文:
Interesting question. To answer the question, you must know a rule, as in this Go tutorial https://go.dev/tour/flowcontrol/12
The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns..
Example 1: Telling the defer function to print value located in a specified memory address.
func deferRun() {
num := 1
numAddr := &num //address of variable num in stack memory, 0xc000076f38 for example
defer func(intAddr *int){
fmt.Printf("num is %d", *numAddr)
}(numAddr) //Hey Go, when the surrounding function returns, print the value located in this address (numAddr=0xc000076f38)
num = 2 //Now the value located in address 0xc000076f38 is 2
return
}
Output will be 2.
Example 2: Telling the defer function to print a specified value.
func deferRun() {
num := 1
numAddr := &num //address of variable num in stack memory, 0xc000076f38 for example
defer fmt.Printf("num is %d", *numAddr) //Hey Go, when the surrounding function returns, print the this value *numAddr (*numAddr is 1 for now)
num = 2 //Now the value located in address 0xc000076f38 is 2 but you told the defer function to print 1 before
return
}
Output will be 1.
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论