Re: [go-nuts] Variable declaration inside loop

[David Symonds]

On Sun, May 19, 2013 at 11:05 AM, Tobia <tobia.c...@gmail.com> wrote:

> But the following surprised me. I expected it to "fail" (print five 5),
> instead it prints from 0 to 4:
>
> for i := 0; i < 5; i++ {
> x := i
> go func() {
> time.Sleep(time.Millisecond)
> fmt.Println(x)
> }()
> }
> time.Sleep(2 * time.Millisecond)

Your "x := i" line is declaring a new variable for each iteration of
the loop, so each closure captures a new x.

[Péter Szilágyi]

Since you're declaring the x inside a block, by the language specs those 5 x-es will be completely different. The optimization is actually noticing that you can reuse it (not in this case).

On Sun, May 19, 2013 at 8:05 PM, Tobia <tobia.c...@gmail.com> wrote:

Hi

I was playing around with closures and I noticed something curious.

First, the obvious behavior. This prints five 5, as expected*:

for i := 0; i < 5; i++ {

go func() {

time.Sleep(time.Millisecond)

fmt.Println(i)

}()

}

time.Sleep(2 * time.Millisecond)

http://play.golang.org/p/WlCESYk7UP

(* There is only one "i" variable in memory, the closure merely captures it. By the time the five goroutines stop sleeping, the single "i" already contains 5.)

This prints the numbers from 0 to 4 (in unpredictable order), again as expected:

for i := 0; i < 5; i++ {

go func(x int) {

time.Sleep(time.Millisecond)

fmt.Println(x)

}(i)

}

time.Sleep(2 * time.Millisecond)

http://play.golang.org/p/Hsf9yib9Cc

But the following surprised me. I expected it to "fail" (print five 5), instead it prints from 0 to 4:

for i := 0; i < 5; i++ {

x := i

go func() {

time.Sleep(time.Millisecond)

fmt.Println(x)

}()

}

time.Sleep(2 * time.Millisecond)

http://play.golang.org/p/CE6q9Bs9xi

Why does a single (lexical) variable declaration cause many memory allocations? Would it always do so, or is this a case of the compiler being "smart" and noticing that the variable is being captured? More importantly, is this specified in the reference docs? Can it be relied upon?

-Tobia

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