Where is my type?

[cpu...@gmail.com]

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 :)

[burak serdar]

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.

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[cpu...@gmail.com]

It's worth noting that before the Unmarshal v is {}(main.S) and afterwards {}(map[string]interface{}) 

[cpu...@gmail.com]

> 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?

[cpu...@gmail.com]

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