On Mon, Sep 21, 2020 at 9:34 AM 'Axel Wagner' via golang-nuts
<golang-nuts@googlegroups.com> wrote:
>
> The evaluation order is defined here:
> https://golang.org/ref/spec#Order_of_evaluation
> The important part is that the order of evaluation in a return statement is
> only defined for function calls, method calls and communication statements,
> but not in relation to other operations. So, in
> return intNumber, setInteger(&intNumber)
> It is not defined whether `intNumber` or `setInteger(&intNumber)` is
> evaluated first, as the former is none of those. I can't really tell you why,
> but it has been discussed a couple of times, you might find something using
> the search function on golang-nuts.
>
> If you want reliable behavior, you should assign the result first
> var intNumber Integer
> err := setInteger(&intNumber)
> return intNumber, err
This flexibility is in the language to permit better code
optimization. The compiler can choose how to order memory loads and
function calls. It is not required to do a memory load, save it
somewhere, and then do the function call.
Of course, this does have the downside that different compilers will
produce different behavior for the same code. So far we've decided
that that is OK.
My personal attitude is if a single statement writes to a variable
other than by assigning to it directly, and the statement also reads
from that same variable, then the program is already confusing. While
we could lock down a specific order of evaluation, that won't help the
fact that the program is hard to read and hard to understand. So I
don't see a good argument for forcing many well written programs to
run very slightly slower in order to make poorly written programs
behave consistently. But I understand that other people feel
differently.
Ian
> On Mon, Sep 21, 2020 at 6:09 PM cs.ali...@gmail.com <cs.alikoyu...@gmail.com>
> wrote:
>>
>> Why does the compiler change evaluation order of return when adding a new
>> field to a struct? I didn't check the compiler output, I only guess it
>> somehow changes the order.
>> play
>> package main
>> import (
>> "fmt"
>> )
>> type Number interface {
>> Value() int
>> }
>> type Integer struct {
>> value int
>> additional int // remove this for different result
>> }
>> func (integer Integer) Value() int {
>> return integer.value
>> }
>> func main() {
>> number, _ := getNumber()
>> fmt.Printf("number val: %d\n", number.Value())
>> }
>> func getNumber() (Number, error) {
>> var intNumber Integer
>> return intNumber, setInteger(&intNumber)
>> }
>> func setInteger(num *Integer) error {
>> num.value = 2
>> return nil
>> }
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "golang-nuts" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to golang-nuts+unsubscr...@googlegroups.com.
>> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/golang-nuts/b7077f03-b0f4-454a-a587-ab9757164409n%40googlegroups.com.
>
> --
> You received this message because you are subscribed to the Google Groups
> "golang-nuts" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to golang-nuts+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/golang-nuts/CAEkBMfHsSFfjPzoOTVz7Q4fE%3DziCXhA5mbUWzSkQ5DADMyzWFw%40mail.gmail.com.
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/CAOyqgcUAEwdMHrNaLKvDyiPEBdXNH8uQ4Uq2WY77s2QFOR8-uA%40mail.gmail.com.
