英文:
Enclosed Variable Overridden In Loop
问题
我正在尝试构建一个包含封闭变量（在这种情况下是一个字符串）的函数数组，但是我得到了一些意外的输出。我猜想我得到这个输出的原因是每次迭代后，被附加的函数文字实际上是一个指向正在更改的代码的指针。
有没有办法使用new()或make()创建一个函数类型，以便append()在每次迭代时都获得一个不同的函数实例？
package main
import "log"
var functions []func()
func main() {
for _, s := range [...]string{"goodbye", "cruel", "world"} {
functions = append(functions, func() {
log.Println(s)
})
}
for _, f := range functions {
f()
}
}
输出：
2014/11/23 18:13:16 world
2014/11/23 18:13:16 world
2014/11/23 18:13:16 world
英文:
I am trying to build an array of functions that contain an enclosed variable (in this case a string) but I was getting some unexpected output. I figure the reason I am getting this output is because the func literal being appended is actually a pointer to code that is being changed after each iteration.
Is there a way to new() or make() a function type so that append() will get a different function instance per iteration instead?
package main
import "log"
var functions []func()
func main() {
for _, s := range [...]string{"goodbye", "cruel", "world"} {
functions = append(functions, func() {
log.Println(s)
})
}
for _, f := range functions {
f()
}
}
Outputs:
2014/11/23 18:13:16 world
2014/11/23 18:13:16 world
2014/11/23 18:13:16 world
答案1
得分: 3
每次循环迭代都使用相同的变量s实例，因此每个闭包共享该单个变量。为了将s的当前值绑定到每个闭包中，需要修改内部循环以在每次迭代时创建一个新变量。例如，
package main
import "log"
var functions []func()
func main() {
for _, s := range [...]string{"goodbye", "cruel", "world"} {
s := s // 创建新的s
functions = append(functions, func() {
log.Println(s)
})
}
for _, f := range functions {
f()
}
}
输出：
2009/11/10 23:00:00 goodbye
2009/11/10 23:00:00 cruel
2009/11/10 23:00:00 world
参考资料：
What happens with closures running as goroutines?
Captured Closure (for Loop Variable) in Go
英文:
Each iteration of the loop uses the same instance of the variable s, so each closure shares that single variable. To bind the current value of s to each closure as it is launched, one must modify the inner loop to create a new variable each iteration. For example,
package main
import "log"
var functions []func()
func main() {
for _, s := range [...]string{"goodbye", "cruel", "world"} {
s := s // create new s
functions = append(functions, func() {
log.Println(s)
})
}
for _, f := range functions {
f()
}
}
Output:
<pre>
2009/11/10 23:00:00 goodbye
2009/11/10 23:00:00 cruel
2009/11/10 23:00:00 world
</pre>
References:
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