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 <cpui...@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+unsubscr...@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/CAMV2RqrUtoauyu8dHGrrVppY%3DeTVP7dCZkb%2Br_Ax696UuJy6dQ%40mail.gmail.com.
