re: solving: panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

[Alexander Mills]

I am trying to get an answer to my github gist Q:

https://gist.github.com/ORESoftware/9253433dc0f5af52df8eff7c26e689a0

if anyone knows if this is possible in Golang v1.13+ please lmk

[Tamás Gulácsi]

Yes, I know several ways.

[Alexander Mills]

it looks like this way works:

https://stackoverflow.com/a/43918797/12211419

know of any others?

[Tamás Gulácsi]

Yup, that's one - get the address of the field and get that.

Another is to declare a similar (memory and type-wise) structure with exported fields,

and unsafe-type-cast it: https://play.golang.org/p/z0QG-CP_MSL

[Axel Wagner]

No need for unsafe:

https://play.golang.org/p/7cTejtO-I1z

You can't *modify* the field, but you can read it just fine. In fact, that's how fmt does it as well:

https://play.golang.org/p/cuFQMsvLmaZ

I suspect you are seeing the error you are seeing because you are either trying to modify it (calling one of the Set* methods of reflect.Value) or use reflect.Value.Interface, which I guess isn't allowed because it might enable you to call methods on that unexported field value and modifying it or something?

https://play.golang.org/p/b2fzyZHZWJs

Either way, if you switch on the reflect.Value.Kind() and use the .String/.Int/.Bool etc., you can read the value at least.

--
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/836756d6-0654-4341-8a2e-af57ff07a231%40googlegroups.com.

[Axel Wagner]

(then again, the fact that this works seems slightly concerning to me: https://play.golang.org/p/phWQ83wPQtx)

[roger peppe]

On Sun, 9 Feb 2020 at 00:19, 'Axel Wagner' via golang-nuts <golan...@googlegroups.com> wrote:

(then again, the fact that this works seems slightly concerning to me: https://play.golang.org/p/phWQ83wPQtx)

Oh, that's interesting! Is this the only way that you can use reflect (without unsafe) to modify the contents of unexported fields?

I think it might be.

[Axel Wagner]

It's reflect, so I don't feel confident in making any absolute statements (we've talked about it a bunch; I just find the implications of that API far too unwieldy to handle), but I tend to agree. You need a reference value and Bytes() seems the only typed reference value you can get out of reflect.Value without using Interface() and type-assertion. I don't think this can be fixed either - not just because of compatibility, but also because it would impede fmt.Print*.

[Ian Lance Taylor]

On Mon, Feb 10, 2020 at 4:15 AM 'Axel Wagner' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> It's reflect, so I don't feel confident in making any absolute statements (we've talked about it a bunch; I just find the implications of that API far too unwieldy to handle), but I tend to agree. You need a reference value and Bytes() seems the only typed reference value you can get out of reflect.Value without using Interface() and type-assertion. I don't think this can be fixed either - not just because of compatibility, but also because it would impede fmt.Print*.

https://golang.org/issue/27727.

Ian

> On Mon, Feb 10, 2020 at 11:46 AM roger peppe <rogp...@gmail.com> wrote:
>>
>> On Sun, 9 Feb 2020 at 00:19, 'Axel Wagner' via golang-nuts <golan...@googlegroups.com> wrote:
>>>
>>> (then again, the fact that this works seems slightly concerning to me: https://play.golang.org/p/phWQ83wPQtx)
>>
>>
>> Oh, that's interesting! Is this the only way that you can use reflect (without unsafe) to modify the contents of unexported fields?
>> I think it might be.
>>

> --
> 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/CAEkBMfG%3Di%2BPgKBfsNPtWNZXMzQV2AAqk%3DnQuo7Oi_pabXcJQEA%40mail.gmail.com.

[roger peppe]

On Tue, 11 Feb 2020 at 03:38, Ian Lance Taylor <ia...@golang.org> wrote:

On Mon, Feb 10, 2020 at 4:15 AM 'Axel Wagner' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> It's reflect, so I don't feel confident in making any absolute statements (we've talked about it a bunch; I just find the implications of that API far too unwieldy to handle), but I tend to agree. You need a reference value and Bytes() seems the only typed reference value you can get out of reflect.Value without using Interface() and type-assertion. I don't think this can be fixed either - not just because of compatibility, but also because it would impede fmt.Print*.

https://golang.org/issue/27727.

Ah, thanks! I'd missed that. I added a comment there FWIW.

[Rohit Basu]

// A use case to avoid:: panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

```

package main

import (
    "fmt"
    "reflect"
    "strings"
    "unsafe"
)

type Person1 struct {
    W3ID string
    Name string
}

type Address1 struct {
    city    string
    country string
}

type User1 struct {
    name      string
    age       int
    address   Address1
    manager   Person1
    developer Person1
    tech      Person1
}

func showDetails(load, email interface{}) {
    //fmt.Println("The values in the first argument are :")
    if reflect.ValueOf(load).Kind() == reflect.Struct {
        typ := reflect.TypeOf(load)
        value := reflect.ValueOf(load)
        value2 := reflect.New(value.Type()).Elem() // #1 For struct, not addressable create a copy With Elements.
        value2.Set(value)                          //#2 Value2 is addressable and can be set
        for i := 0; i < typ.NumField(); i++ {
            //fmt.Println(value.Field(i).Kind(), "---------")
            if value.Field(i).Kind() == reflect.Struct {
                rf := value2.Field(i)
                rf = reflect.NewAt(rf.Type(), unsafe.Pointer(rf.UnsafeAddr())).Elem()
                irf := rf.Interface()
                typrf := reflect.TypeOf(irf)
                nameP := typrf.String()
                if strings.Contains(nameP, "Person") {
                    //fmt.Println(nameP, "FOUND !!!!!!! ")
                    for j := 0; j < typrf.NumField(); j++ {
                        //fmt.Println(j)
                        re := rf.Field(j)
                        nameW := typrf.Field(j).Name
                        //fmt.Println(nameW)
                        if strings.Contains(nameW, "W3ID") {
                            //fmt.Println(nameW)
                            valueW := re.Interface()
                            fetchEmail := valueW.(string)
                            //fmt.Println(fetchEmail)
                            if fetchEmail == email {
                                fmt.Println(fetchEmail, " MATCH!!!!")
                            }
                        }
                    }
                }
                //fmt.Println(rf.Interface(), "++=====")
                showDetails(irf, email)
            } else {
                // fmt.Printf("%d.Type:%T || Value:%#v\n",
                //     (i + 1), value.Field(i), value.Field(i))

                // fmt.Println("Kind is ", value.Field(i).Kind())
            }
        }
    }
}

func main() {
    iD := "ts...@in.org.com"

    load := User1{
        name: "John Doe",
        age:  34,
        address: Address1{
            city:    "New York",
            country: "USA",
        },
        manager: Person1{
            W3ID: "jB...@in.org.com",
            Name: "Bualt",
        },
        developer: Person1{
            W3ID: "ts...@in.org.com",
            Name: "Sumi",
        },
        tech: Person1{
            W3ID: "lP...@in.org.com",
            Name: "Paul",
        },
    }

    showDetails(load, iD)
}

```