encoding/json: Unmarshal into *interface{} that contains specific value

[gopherbot]

by Lytvynov.A.V:

What steps will reproduce the problem?
If possible, include a link to a program on play.golang.org.
http://play.golang.org/p/PQ7zcqcdN-
Pass pointer to an interface{} value that contains specific struct, instead of
interface{} value that contains pointer to specific struct.

What is the expected output?
main.B{Key:"", Val:""}
main.B{Key:"one", Val:"1"}

What do you see instead?
main.B{Key:"", Val:""}
map[string]interface {}{"Key":"one", "Val":"1"}

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
linux

Which version are you using?  (run 'go version')
go version devel +b4c37131e846 Mon Oct 21 19:01:24 2013 -0400 linux/amd64
also works on release version

Please provide any additional information below.
Obvious solution to the code above would be http://play.golang.org/p/ffp11TvZoe
But should it *only* work that way?

[adg]

Comment 1:

The first version of the unmarshal function is useless. You're just passing in a struct
value. Even if it "worked", the unmarshal function would populate a copy of that value
and you could never retrieve the contents of the JSON blob.

[gopherbot]

Comment 2 by Lytvynov.A.V:

Indeed, but that was just a simplified example demonstrating the behavior.
Here are similar examples:
http://play.golang.org/p/nhpFM4roxD
and
http://play.golang.org/p/PDwJF7Kcxj

[adg]

Comment 3:

The issue is that the concrete value of the interface{} value is just a value, and is
not addressable, and so the JSON package can't modify it.
Change the line
    a := &A{B: B{}}
to
    a := &A{B: &B{}}
and you'll see that it works.
Here is a reduced example that demonstrates the difference:
  http://play.golang.org/p/AyC8gSZh-d

Status changed to WorkingAsIntended.

[gopherbot]

Comment 4 by Lytvynov.A.V:

Ah, so it's about addressability.
That makes it clearer. Thanks!