cannot assign to struct field in map

Question

I would like to set a default value and if this is not set in a structure, I would like to set it. It's a bit confusing, but please take a look to the (simplified) code:

package main

import "log"

type Something struct {
    A string
    B map[string]Type
    C Epyt
}
type Type struct {
    A Epyt
    B string
}

type Epyt struct {
    A string
    B string
}

func main() {
    var a Something
    a.A = "Test A (Something)"
//  var a.B["one"] Type
    a.B["one"].A.A = a.B["one"].A.A
    a.B["one"].A.A = "Test A ([one]Type.Epyt)"
    a.B["two"].A.A = "Test A ([two]Type.Epyt)"  
    a.C.A = "Test A (Epyt)"
    a.C.B = "Test B (Epyt)"

    for i := range a.B {
        if a.B[i].A.B == "" {
            a.B[i].A.B = a.C.B
        }
    }
    log.Printf("%+v", a)
}

I'm working with viper to unmarshal a config file, therefor I'm not able to use pointers (or am I wrong?).

The error I'm getting, is cannot assign to struct field *** in map.

I found that this is an old bug in go lang still not corrected.

I've read Why do I get a "cannot assign" error when setting value to a struct as a value in a map? and Golang: I have a map of structs. Why can't I directly modify a field in a struct value? but as I said, I can't operate with pointers (or?), therefor please don't mark this as duplicate as it is not!

If someone has an idea on how to do this right, please help!

a.B is map as such you can not reference a field of it. If you want to access a field of an object in the map you will need to use a key like you do above a.B["two"].A.B = a.CB. If you want to do this to every member of the map you should use a loop — Benjamin Kadish
– Benjamin Kadish, Commented May 7, 2018 at 19:59
Thank you, Benjamin! I've corrected the code in the question as this was a typo in writing this simple code. Still need help! — Boštjan Bele
– Boštjan Bele, Commented May 7, 2018 at 20:08
Where are you getting the error? a.C.A = "Test A (Epyt)" is definitely wrong, but doesn't have anything to do with maps. — Vitruvie
– Vitruvie, Commented May 7, 2018 at 20:19
@BoštjanBele What makes you think you cannot unmarshal into a map of pointers? Did you try it? — mkopriva
– mkopriva, Commented May 7, 2018 at 20:21
This is a duplicate of stackoverflow.com/questions/32751537/… and others. These other questions can also be made to work by using pointers. There is not a bug to correct in Go. Map values are not addressable. — user5728991
– user5728991, Commented May 7, 2018 at 20:21

Boštjan Bele · Accepted Answer · 2018-05-08 13:54:44Z

1

I played around a bit and got it working! I thought there is an error, because if I use printf over the whole structure, I get the address instead of the value back. Sorry for the inconvenience!

As @mkopriva told me, I tried to use pointers. After some errors I got it to work so far! Sorted out this brain bug ;)

Thank you again!

answered May 8, 2018 at 13:54

Boštjan Bele

231 silver badge5 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Edson Medina · Accepted Answer · 2018-05-08 18:31:01Z

1

You haven't initialised the map.

Try

var a Something
a.B = make(map[string]Type)

Also, none of the structures you're referring to exist.

Ie, instead of:

a.B["one"].A.A = ...

You should do:

a.B["one"] = Type{
    A: Epyt{
        A: "test",
        B: "foo",
    },
    B: "something",
}

edited May 8, 2018 at 18:31

answered May 8, 2018 at 18:25

Edson Medina

10.3k4 gold badges43 silver badges53 bronze badges

Collectives™ on Stack Overflow

cannot assign to struct field in map

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related