循环指针

Question

请考虑摘要https://play.golang.org/p/GnhA1Tgw4sz，它是我遇到的问题的简化版本。我最初的问题是尝试将UDP消息发送到阵列中的目标，但我发现均匀分配存在问题。

另外，代码：

package main

import (
    "fmt"
    "time"
)

var (
    dests = [...]string{"word1", "word2", "word3", "word4", "word5", "word6", "word7", "word8"}
)

func main() {
    fmt.Println("Hello!")
    fmt.Println("dests", dests)

    for _, dest := range dests {
        fmt.Println("dest is", dest)

        go func(dest_ptr *string) {
            fmt.Println("Trying", *dest_ptr, dest_ptr)
        }(&dest)

    }

    time.Sleep(200 * time.Second)
}

我跑步的时候

Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140
Trying word8 0x40c140

注意这里，我一直在打word8，可能是因为&dest被覆盖了。我知道没有指针就不会有这个问题，如demodd @ https://play.golang.org/p/ZD9JIuvdypJ。

我想使用指针，因为最初的问题需要它。什么是实现此目标的正确方法？

来自C，这是一个很好的问题！据我了解，Go不会为每次迭代创建一个新变量，以后可能会解决此问题。

这是两个解决方法，

$ cat fixa.go
package main

import (
        "fmt"
        "time"
)

var (
        dests = [...]string{"word1", "word2", "word3", "word4", "word5", "word6", "word7", "word8"}
)

func main() {
        fmt.Println("Hello!")
        fmt.Println("dests", dests)

        for i := range(dests){
                go func(dest_ptr * string){
                        fmt.Println("Handling", * dest_ptr, dest_ptr)
                }(&dests[i])

        }

        time.Sleep(200 * time.Second)
}

和，

$ cat fixb.go
package main

import (
        "fmt"
        "time"
)

var (
        dests = [...]string{"word1", "word2", "word3", "word4", "word5", "word6", "word7", "word8"}
)

func main() {
        fmt.Println("Hello!")
        fmt.Println("dests", dests)

        for _, dest := range dests {
                dest := dest //  create a new variable, using a declaration style that may seem odd but works fine in Go: 
                go func(dest_ptr *string) {
                        fmt.Println("Trying", *dest_ptr, dest_ptr)
                }(&dest)
        }

        time.Sleep(200 * time.Second)
}

Answer 1

问题中的代码将局部变量的地址传递dest给goroutine。该局部变量在循环的每次迭代中设置。

通过将数组元素的地址传递给goroutine来解决：

for i := range dests {
    fmt.Println("dest is", dests[i])
    go func(dest_ptr *string) {
        fmt.Println("Trying", *dest_ptr, dest_ptr)
    }(&dests[i])
}

另一种方法是在循环的每次迭代中创建一个变量：

for _, dest := range dests {
    dest := dest  // <-- create new variable inside the loop
    fmt.Println("dest is", dest)
    go func(dest_ptr *string) {
        fmt.Println("Trying", *dest_ptr, dest_ptr)
    }(&dest)
}