I wrote a simple test. Sure enough it fails, and it reports a data race.
package main
import (
"log"
"sync"
)
type S struct {
sync.Mutex
index int
values [128]int
}
func (s *S) mutate() {
s.Lock();
defer s.Unlock();
s.index++;
for i:=0; i< 128; i++ {
s.values[i]=s.index;
}
}
func (s S) validate() {
for i:=0;i<128;i++ {
if s.values[i]!=s.index {
log.Fatal("mismatch error")
}
}
}
func doit(s *S) {
for {
s.mutate()
s.validate()
}
}
func main() {
var s S
var wg sync.WaitGroup
wg.Add(1)
for i:=0;i<64;i++ {
go doit(&s)
}
wg.Wait()
}
In fact, you get a linter warning, because of the copy of the mutex in calling
the value method - since it knows it should be a reference.
> On Oct 7, 2024, at 5:30 PM, Robert Engels <reng...@ix.netcom.com> wrote:
>
> I am fairly certain if you mix pointer and receiver methods and the receiver
> methods mutate - even if you synchronize those you will get a data race
> calling the value methods. It must afaik as the runtime/compiler has no
> implicit synchronization when creating the copies. That is a data race.
>
>> On Oct 7, 2024, at 5:10 PM, Axel Wagner <axel.wagner...@googlemail.com>
>> wrote:
>>
>>
>> My argument had nothing to do with synchronization.
>>
>> FTR I find the synchronization argument also extremely dubious. By that
>> argument, you also can't pass the address to a local variable to another
>> function, when using it as a value elsewhere. It's a weird argument to make.
>> time.Time uses a mix of pointer- and value receivers and IMO no one can make
>> a serious argument that this would expose programs to risks of data races.
>>
>> But to repeat my actual argument in favour of (sometimes) mixing receiver
>> kinds:
>> 1. It is totally reasonable to use some types as values.
>> 2. Such types, intended to be used as values, will need to use
>> value-receivers for some methods, as otherwise their value-version does not
>> implement certain interfaces (methods are not promoted from pointer to value
>> types). Like fmt.Stringer, for example. And
>> 3. such types still need to sometimes use pointer-receivers, to implement
>> functionalities like unmarshalling.
>>
>> time.Time is a standard library example of such a type. I also provided an
>> example for an "enum-like" type implementing flag.Value.
>>
>> On Mon, 7 Oct 2024 at 23:57, Robert Engels <reng...@ix.netcom.com
>> <mailto:reng...@ix.netcom.com>> wrote:
>> I am pretty sure it is immaterial. If the object isn’t immutable any copy or
>> mutation operation needs to be synchronized.
>>
>> But the problem afaik is that you can’t control synchronization when the
>> object is copied for a value receiver - which means you cant properly
>> synchronize when you have pointer and value receivers unless you do it
>> externally (which is a huge pain to do everywhere).
>>
>>> On Oct 7, 2024, at 4:43 PM, 'Axel Wagner' via golang-nuts
>>> <golang-nuts@googlegroups.com <mailto:golang-nuts@googlegroups.com>> wrote:
>>>
>>>
>>> No offence, but I made an argument. You don't have to agree with the
>>> argument and it might be wrong. But to convince me, at least, that argument
>>> would need to actually be referenced.
>>>
>>> I gave reasons why, in my opinion, *not* mixing value and pointer receivers
>>> sometimes leads to incorrect code. So as far as I'm concerned (until
>>> someone tells me my reasons are wrong) Goland's linter simply encourages
>>> you to write bad code. It would not be the first time that I strongly
>>> disagree with the recommendations of an IDE. Goland in particular has a
>>> history of making, in my opinion, pretty questionable decisions.
>>>
>>> On Mon, 7 Oct 2024 at 22:39, Cleberson Pedreira Pauluci
>>> <pauluci.cleber...@gmail.com <mailto:pauluci.cleber...@gmail.com>> wrote:
>>> Many places and books I've read generally say: If a function needs to
>>> update a variable, or if an argument is so large that we want to avoid
>>> copying it, we should pass the pointer. Same for methods (pointer
>>> receiver). (The Go programming language book).
>>>
>>> About mixing "value receiver" and "pointer receiver". Even the IDE
>>> complains about this and recommends following the Go documentation. (Goland)
>>>
>>>
>>> Em segunda-feira, 7 de outubro de 2024 às 15:15:25 UTC-3, burak serdar
>>> escreveu:
>>> Mixing pointer and value receivers can be race-prone, because of the
>>> copying involved in passing value receivers.
>>>
>>> On Mon, Oct 7, 2024 at 12:03 PM 'Axel Wagner' via golang-nuts
>>> <golan...@googlegroups.com <>> wrote:
>>> >
>>> > To be honest, I always found this recommendation a little bit strange,
>>> > personally.
>>> >
>>> > I'll note that the standard library does not really keep to this either.
>>> > For example, time.Time.UnmarshalText (obviously) has a pointer-receiver,
>>> > while almost all other methods on time.Time have a value receiver.
>>> > And if you implement flag.Value, the Set method obviously needs a pointer
>>> > receiver, but if the String method has one as well, it won't print
>>> > properly when used as a value. In basically every implementation of
>>> > flag.Value I've ever written, String needed a value receiver, while Set
>>> > needed a pointer receiver.
>>> >
>>> > I understand the basic idea of the advice, that if a type keeps state
>>> > that is manipulated via methods, then it should generally be passed
>>> > around as a pointer, so giving all the methods a pointer-receiver works
>>> > well. But if a type *is* intended to be used as a value (like time.Time
>>> > or Enum in my example) then you will almost certainly end up with a mix
>>> > of receiver kinds - as soon as you want to add any form of
>>> > de-serialization to it. So "don't mix receiver kinds" seems like
>>> > misleading advice to me.
>>> >
>>> > On Mon, 7 Oct 2024 at 19:44, Ian Lance Taylor <ia...@golang.org <>>
>>> > wrote:
>>> >>
>>> >> On Mon, Oct 7, 2024 at 10:29 AM Ken Lee <ken.lee....@gmail.com <>>
>>> >> wrote:
>>> >> >
>>> >> > ---
>>> >> > There is a consideration to make, though: historically it has been
>>> >> > considered bad form in Go to give a type a mix of value and pointer
>>> >> > receivers in methods without a very specific reason for doing so.
>>> >> > ---
>>> >> >
>>> >> > Is this still the case now? As in 2024.
>>> >>
>>> >> As a general guideline, yes.
>>> >>
>>> >> https://go.dev/wiki/CodeReviewComments#receiver-type
>>> >> <https://go.dev/wiki/CodeReviewComments#receiver-type>
>>> >>
>>> >> Ian
>>> >>
>>> >>
>>> >>
>>> >> > On Sunday 13 January 2013 at 7:03:29 am UTC+8 Kevin Gillette wrote:
>>> >> >>
>>> >> >> Indeed. In addition to implicit dereferencing for value receivers,
>>> >> >> the reverse also works as well: anything that is addressable
>>> >> >> (including 'value' variables on the stack, or a field of element of
>>> >> >> anything that's addressable) will implicitly be addressed when a
>>> >> >> pointer-receiver method is called on them (though you must explicitly
>>> >> >> use the address operator when you need to pass value variables as
>>> >> >> pointers).
>>> >> >>
>>> >> >> There is a consideration to make, though: historically it has been
>>> >> >> considered bad form in Go to give a type a mix of value and pointer
>>> >> >> receivers in methods without a very specific reason for doing so. The
>>> >> >> typical justification is that a small struct in a getter method might
>>> >> >> as well have a value receiver even though the corresponding setter
>>> >> >> method uses a pointer receiver; this, however, can lead to confusion
>>> >> >> on the part of the app programmer if they start out using only the
>>> >> >> read-only methods upon what turns out to be a value-copy of the
>>> >> >> original (but hey, it compiled and seems to work, so it must be
>>> >> >> correct) -- when use of pointer-receiver methods don't seem to
>>> >> >> produce the documented changes in the original, it can be difficult
>>> >> >> to debug.
>>> >> >>
>>> >> >>
>>> >> >> On Saturday, January 12, 2013 3:17:16 PM UTC-7, Dave Collins wrote:
>>> >> >>>
>>> >> >>> On Saturday, January 12, 2013 3:52:35 PM UTC-6, Taric Mirza wrote:
>>> >> >>>>
>>> >> >>>> Thanks! Works like a charm and is helping cleaning up my code a
>>> >> >>>> ton.
>>> >> >>>>
>>> >> >>>> One other question, this is really more about coding style:
>>> >> >>>>
>>> >> >>>> In the case where you manipulate members of the struct, then using
>>> >> >>>> pointers as in your example is the way to go.
>>> >> >>>>
>>> >> >>>> But, you have a choice for functions that just read values from the
>>> >> >>>> struct instead of manipulating it. Is there a best practice coding
>>> >> >>>> style here, between dereferencing the struct and then using that,
>>> >> >>>> or
>>> >> >>>> dereferencing each member of the struct as you go? eg:
>>> >> >>>>
>>> >> >>>> // A:
>>> >> >>>>
>>> >> >>>> laser := worldobj.(*Laser)
>>> >> >>>> fmt.Printf("%0.4f,%0.4f", (*laser).x, (*laser).y)
>>> >> >>>>
>>> >> >>>> versus
>>> >> >>>>
>>> >> >>>> // B:
>>> >> >>>>
>>> >> >>>> laser := *(worldobj.(*Laser))
>>> >> >>>> fmt.Printf("%0.4f,%0.4f", laser.x, laser.y)
>>> >> >>>>
>>> >> >>>>
>>> >> >>>> I'm kind of torn. I would imagine A) has slightly better
>>> >> >>>> performance, and doesn't require any code-rework if you later on
>>> >> >>>> need
>>> >> >>>> to manipulate the struct.
>>> >> >>>>
>>> >> >>>> On the other hand, B) is more readable since you don't have to look
>>> >> >>>> at
>>> >> >>>> pointers all over the place, just on one line.
>>> >> >>>
>>> >> >>>
>>> >> >>> Actually, you don't need to dereference at all. Go automatically
>>> >> >>> handles this for you.
>>> >> >>>
>>> >> >>> See this example: http://play.golang.org/p/ANaKaFSQLn
>>> >> >>> <http://play.golang.org/p/ANaKaFSQLn>
>>> >> >>>
>>> >> > --
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>>> >> > https://groups.google.com/d/msgid/golang-nuts/03df7dce-5c48-44a3-bc3c-851ded2a1f08n%40googlegroups.com
>>> >> >
>>> >> > <https://groups.google.com/d/msgid/golang-nuts/03df7dce-5c48-44a3-bc3c-851ded2a1f08n%40googlegroups.com>.
>>> >> >
>>> >>
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