c := getX()
c.x = 1
vs
getX().x = 1
On Wed, Nov 20, 2019 at 2:32 PM Robert Engels <reng...@ix.netcom.com> wrote:
> I must of misread something, because this code works fine:
>
> package main
>
> import "fmt"
>
> type X struct {
> x int
> }
>
> func getX() X {
> var x X
> return x
> }
>
> func main() {
> c := getX()
> c.x = 1
> fmt.Println(c)
> }
>
> and is what I would expect to happen. Why is this not the same behavior in
> the other code?
>
>
>
>
>
> -----Original Message-----
> >From: Robert Engels <reng...@ix.netcom.com>
> >Sent: Nov 20, 2019 2:25 PM
> >To: Ian Lance Taylor <i...@golang.org>, Orson Cart <
> objectivedynam...@gmail.com>
> >Cc: golang-nuts <golang-nuts@googlegroups.com>
> >Subject: Re: [go-nuts] Difference with function returning struct and
> function returning pointer to struct
> >
> >
> >To be honest, I find this very confusing, especially the statement
> "Changing a field in that copy won't change anything." because even if I
> have a copy, I should be able to change the fields, and the changes would
> be there for the scope, and if I returned the copy, whether by address or
> copy, the changes would be retained.
> >
> >-----Original Message-----
> >>From: Ian Lance Taylor <i...@golang.org>
> >>Sent: Nov 20, 2019 11:55 AM
> >>To: Orson Cart <objectivedynam...@gmail.com>
> >>Cc: golang-nuts <golang-nuts@googlegroups.com>
> >>Subject: Re: [go-nuts] Difference with function returning struct and
> function returning pointer to struct
> >>
> >>On Wed, Nov 20, 2019 at 9:41 AM Orson Cart <objectivedynam...@gmail.com>
> wrote:
> >>>
> >>> I'm a newbie to Go having used C and C++ in the past and I'm puzzled
> about something I've just seen.
> >>>
> >>> In the following code getEmployee returns a pointer to a struct. The
> return value is then dereferenced. This code compiles and runs fine:
> >>>
> >>> package main
> >>>
> >>> type employee struct {
> >>> ID int
> >>> name string
> >>> }
> >>>
> >>> var dilbert employee
> >>>
> >>> func main() {
> >>>
> >>> getEmployee().name = "dilbert"
> >>> }
> >>>
> >>> func getEmployee() *employee {
> >>> return &dilbert
> >>> }
> >>>
> >>> However if I modify getEmployee as follows, I get a compilation error:
> >>>
> >>> func getEmployee() employee {
> >>> return dilbert
> >>> }
> >>>
> >>> The error is: cannot assign to getEmployee().name
> >>>
> >>> I assume that it revolves around "assignability" but I'm struggling to
> understand why.
> >>>
> >>> Can anyone tell me why this is?
> >>
> >>There are two kinds of answers.
> >>
> >>One is that the spec only permits setting fields of a struct that is
> >>addressable, and the result of a function call is not addressable.
> >>See https://golang.org/ref/spec#Assignments and
> >>https://golang.org/ref/spec#Address_operators .
> >>
> >>The other kind of answer is that if a function returns a pointer to a
> >>struct, a value of type *employee, then it's normal for that something
> >>else to have a copy of that pointer, perhaps some data structure or
> >>global variable. In your simple example that is the global variable
> >>"dilbert". So it makes sense to permit setting a field of the result
> >>of calling getEmployee(), a value of type *employee, as setting that
> >>field will change the global variable "dilbert". But when
> >>getEmployee() does not return a pointer, but just returns a value of
> >>type "employee", then that is a copy of the value in the global
> >>variable. Changing a field in that copy won't change anything. The
> >>assignment will be made, and then the result will be discarded. Since
> >>that operation is useless, the language does not permit it.
> >>
> >>Ian
> >>
> >>--
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> .
> >
> >--
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