Re: [go-nuts] A question about copying
Aha, I see. Thanks for explaining. br. Chr. lørdag den 25. juli 2020 kl. 10.56.35 UTC+2 skrev Jan Mercl: > On Sat, Jul 25, 2020 at 10:09 AM chri...@surlykke.dk > wrote: > > > Is this something I should have deduced from the language spec? > > https://golang.org/ref/spec#Address_operators > > > For an operand x of type T, the address operation generates a > pointer of type *T to x. The operand must be addressable, that is, > either a variable, pointer indirection, or slice indexing operation; > or a field selector of an addressable struct operand; or an array > indexing operation of an addressable array. > > > Note the 'pointer indirection'. > -- 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/32fe07f9-91a3-44d5-be72-9953c431936fn%40googlegroups.com.
Re: [go-nuts] A question about copying
On Sat, Jul 25, 2020 at 10:09 AM chri...@surlykke.dk wrote: > Is this something I should have deduced from the language spec? https://golang.org/ref/spec#Address_operators For an operand x of type T, the address operation generates a pointer of type *T to x. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. Note the 'pointer indirection'. -- 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/CAA40n-WwxyEQ3TTuozCrLMBoDg15SsQ%3DFg2RPXCJ7s_5X2MQqQ%40mail.gmail.com.
Re: [go-nuts] A question about copying
You are probably thinking about code like this:
var f2 = *f1
Which will make a copy, although not because `f1` is dereferenced, but
because `=` was called on a value.
Dereferencing a pointer gives a reference to the same value, taking
address of the same value will produce a pointer to the same value. So
in expression like this:
var f2 = &(*f1)
`&` and `*` cancel each other out and it can be simplified to:
var f2 = f1
Which means "make a copy of pointer". If you take the address of `f1`
and `f2` you'll see that those are indeed different pointers.
If you're thinking about something like `f(*f1)`, then I believe the
function call will make a copy because arguments are passed by values.
`f(f1)` will make a copy too, but a copy of pointer.
Does that make sense?
сб, 25 июл. 2020 г. в 11:09, chri...@surlykke.dk :
>
> When running this program:
>
> package main
>
> import (
> "fmt"
> )
>
> type Foo struct {
> i int
> }
>
> func main() {
> var f1 = {i: 0}
> var f2 = &(*f1)
> f2.i = 1
> fmt.Println(f1, f2)
> }
>
> it yields:
>
> &{1} &{1}
>
> (https://play.golang.org/p/qKtURokUCEW)
>
> I (naively) assumed that the expression
>
> &(*f1)
>
> would, first, create a copy of *f1, and then a pointer to that copy, but
> evidently f2 becomes a pointer to the same struct as f1. Is this something I
> should have deduced from the language spec?
>
> best regards Christian Surlykke
>
>
> --
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Re: [go-nuts] A question about copying
On Sat, 2020-07-25 at 01:09 -0700, chri...@surlykke.dk wrote: > &(*f1) > > would, first, create a copy of *f1, and then a pointer to that copy, > but evidently f2 becomes a pointer to the same struct as f1. Is this > something I should have deduced from the language spec? &(*p) says "give me the address of the thing that is pointed to by p". This is p. If you assign the value of *p to another variable and take the address of that, then you'll get the outcome you were wanting. -- 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/715c99a366a243ff9768a22f96c5a1dc147ace01.camel%40kortschak.io.
[go-nuts] A question about copying
When running this program:
package main
import (
"fmt"
)
type Foo struct {
i int
}
func main() {
var f1 = {i: 0}
var f2 = &(*f1)
f2.i = 1
fmt.Println(f1, f2)
}
it yields:
&{1} &{1}
(https://play.golang.org/p/qKtURokUCEW)
I (naively) assumed that the expression
&(*f1)
would, first, create a copy of *f1, and then a pointer to that copy, but
evidently f2 becomes a pointer to the same struct as f1. Is this something
I should have deduced from the language spec?
best regards Christian Surlykke
--
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