TIL

capturing iteration variables in Go

October 9, 2021 - 1 minutes read - 150 words

I was going through The Go Programming Language and in chapter 5 there was a section Caveat: Capturing Iteration Variables.

The example is like this:

var rmdirs []func()

for _, dir := range tempDirs() {                    // 1
    os.MkdirAll(dir, 0755)                          // 2
    rmdirs = append(rmdirs, func() {
        os.RemoveAll(dir) // NOTE: incorrect!       // 3
    })
}

dir is the same variable. i.e. the address of the variable is same
dir’s value evaluated at this point is different at each iteration
IMPORTANT dir is not evaluated here. Its just being used. It will be evaluated only when the func() is called

The following explanation from Ref[1]:

var rmdirs []func()
tempDirs := []string{"one", "two", "three", "four"}

for _, d := range tempDirs {
    dir := d
    fmt.Printf("dir=%v, *dir=%p\n", dir, &dir)
    rmdirs = append(rmdirs, func() {
        fmt.Printf("dir=%v, *dir=%p\n", dir, &dir)
    })
}

fmt.Println("---")

for _, f := range rmdirs {
    f()
}

explanation

Reference: