Your answer has put me on the right track. Here's the long version: https://stackoverflow.com/questions/48790663/why-value-stored-in-an-interface-is-not-addressable-in-golang
On Wednesday, April 24, 2024 at 6:55:53 PM UTC+2 cpu...@gmail.com wrote: > > In the first case, the interface contains a value of type S, which is > not writable. The value contained in the interface is not addressable. > > Thank you for the quick feedback. > Why is that? Memory for val has been allocated and val is passed by value > (hence copied). Why is that copy not be addressable? > > On Wednesday, April 24, 2024 at 6:51:21 PM UTC+2 burak serdar wrote: > >> In the first case, the interface contains a value of type S, which is >> not writable. The value contained in the interface is not addressable. >> So Unmarshal creates a new map and fills that. In the second case the >> interface contains *S, which is writable, so unmarshal fills it in via >> reflection. >> >> On Wed, Apr 24, 2024 at 10:46 AM cpu...@gmail.com <cpu...@gmail.com> >> wrote: >> > >> > Every time I feel I've come to terms with how Go works I round a corner >> and hit a wall, even after doing this for >5 years. Here's one. >> > >> > Consider this simple code (https://go.dev/play/p/bph5I80vc99): >> > >> > package main >> > >> > import ( >> > "encoding/json" >> > "fmt" >> > ) >> > >> > type S struct { >> > Foo int >> > } >> > >> > func update(v any) { >> > if err := json.Unmarshal([]byte(`{"Foo": 42}`), &v); err != nil { >> > panic(err) >> > } >> > fmt.Printf("%v %T\n", v, v) >> > } >> > >> > func main() { >> > var val S >> > >> > // map[Foo:42] map[string]interface {} >> > update(val) >> > >> > // &{42} *main.S >> > update(&val) >> > } >> > >> > Why would calling by value change the type of the value passed to map? >> I would expect an interface with a dynamic type of main.S, but its >> map[string]interface{}, or json.Unmarshal makes it so. >> > >> > Insights appreciated :) >> > >> > -- >> > 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...@googlegroups.com. >> > To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/980466d2-1686-4b79-aec1-45b592db2caan%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/0010deaa-b3c9-4323-a1f4-d0dc3eec3c78n%40googlegroups.com.
