golang reflect cannot recognize tag from map members

Issue

I want to extract struct’s map members’ tag with reflect, while I found if retrieve member’s value from MapIndex, the type of it will be recognized as "*interface{}" and hence all type information are lost, no mention reflect can extract detail information.

package main

import (
    "fmt"
    "reflect"
)

type Student struct {
    Sname string `MyTag:"student-name"`
}

type Teacher struct {
    Name     string             `MyTag:"teacher-name"`
    Students map[string]Student `MyTag:"teacher-students"`
}

var sam = Teacher{
    Name: "Sam",
    Students: map[string]Student{
        "Sen": {
            Sname: "Sen",
        },
    },
}

func traversalTag(obj interface{}) {
    theType := reflect.TypeOf(obj)
    fmt.Printf("Traversal tag with obj: type %v, value %v\n", theType.String(), obj)

    elem := reflect.TypeOf(obj).Elem()
    for i := 0; i < elem.NumField(); i++ {
        fmt.Printf("Tag name %s, value %s\n", elem.Field(i).Name, elem.Field(i).Tag)
    }
}

func tryMapWithType(students map[string]Student) {
    for key, theValue := range students {
        fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, theValue, &theValue)
        traversalTag(&theValue)
    }
}

func tryMapWithReflect(obj interface{}) {
    reflectMap := reflect.ValueOf(obj)
    for _, key := range reflectMap.MapKeys() {
        theValue := reflectMap.MapIndex(key).Interface()
        fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, theValue, &theValue)
        traversalTag(&theValue) // Will have error
    }
}

func main() {
    tryMapWithType(sam.Students)
    tryMapWithReflect(sam.Students)
}

After run I got following error:

Starting: C:\Users\Mento\go\bin\dlv.exe dap --check-go-version=false --listen=127.0.0.1:50308 from d:\Coding\Golang\demo
DAP server listening at: 127.0.0.1:50308
Key: Sen, Value: {Sen}, value pointer 0xc000044230
Traversal tag with obj: type *main.Student, value &{Sen}
Tag name Sname, value MyTag:"student-name"
Key: Sen, Value: {Sen}, value pointer 0xc0000442c0
Traversal tag with obj: type *interface {}, value 0xc0000442c0
panic: reflect: NumField of non-struct type interface {}

goroutine 1 [running]:
reflect.(*rtype).NumField(0xec2d20)
    C:/Program Files/Go/src/reflect/type.go:1015 +0xc8
main.traversalTag({0xebd9e0, 0xc0000442c0})
    d:/Coding/Golang/demo/demo.go:31 +0x1cb
main.tryMapWithReflect({0xec3d40, 0xc00007a480})
    d:/Coding/Golang/demo/demo.go:48 +0x2c9
main.main()
    d:/Coding/Golang/demo/demo.go:54 +0x38
Process 8716 has exited with status 2
dlv dap (11448) exited with code: 0

Can anyone hint how to get pointer of map members with original type information?

Thanks you,
Mento

Solution

As you know using &theValue resolves to the type *interface{}. The type *interface{} is distinct from the type *Student which is what you are passing in to traversalTag from tryMapWithType.

If you want to pass *Student to traversalTag from tryMapWithReflect you need to create that pointer value using reflection. Plain native Go address operator & just isn’t enough.

When you have a reflect.Value that is addressable all you need to do is to call the .Addr() method to get a pointer to the addressable value, however map elements are not addressable and therefore reflectMap.MapIndex(key) is not addressable. So, unfortunately for you, it’s not possible to do reflectMap.MapIndex(key).Addr().Interface() to get *Student.

So your only option is to use reflection to create a new value of the *Student type, set the pointed-to value to the value in the map, and then return the .Interface() of that.

func tryMapWithReflect(obj interface{}) {
    reflectMap := reflect.ValueOf(obj)
    for _, key := range reflectMap.MapKeys() {
        theValue := reflectMap.MapIndex(key).Interface()

        // allocate a new value of type *Student
        newValue := reflect.New(reflectMap.MapIndex(key).Type())

        // use Elem do dereference *Stunded
        // and then use Set to set the Student to the content of theValue
        newValue.Elem().Set(reflect.ValueOf(theValue))

        fmt.Printf("Key: %v, Value: %v, value pointer %p\n", key, newValue.Elem().Interface(), newValue.Interface())

        // return the newValue *Student
        traversalTag(newValue.Interface())
    }
}

https://go.dev/play/p/pNL2wjsOW5y


Alternatively, just drop the .Elem() from the traversalTag and then you don’t have to pass pointers to it.

func traversalTag(obj interface{}) {
    theType := reflect.TypeOf(obj)
    fmt.Printf("Traversal tag with obj: type %v, value %v\n", theType.String(), obj)

    elem := reflect.TypeOf(obj)
    for i := 0; i < elem.NumField(); i++ {
        fmt.Printf("Tag name %s, value %s\n", elem.Field(i).Name, elem.Field(i).Tag)
    }
}

https://go.dev/play/p/EwJ5e0uc2pd

Answered By – mkopriva

Answer Checked By – Cary Denson (GoLangFix Admin)

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