Get struct field tag using Go reflect package

Question

Is it possible to reflect on a field of a struct, and get a reference to its tag values?

For example:

type User struct {
    name    string `json:name-field`
    age     int
}
// ...
user := &User{"John Doe The Fourth", 20}
getStructTag(user.name)
// ...
func getStructTag(i interface{}) string{
   //get tag from field
   
}

From what I can see, the usual way to do this is to range over typ.NumField(), and then call field.Tag.Get("tagname").

However, in my use-case, it would be much better to not have to pass the whole struct in.

You can also get fields by name. What's wrong with passing the struct? If you don't pass the struct, how do you intend to get the type? — JimB, May 07 '14 at 01:35

score 78 · Accepted Answer · edited Jul 09 '20 at 14:30

78

You don't need to pass in the whole struct, but passing in the value of one of the fields is not sufficient.

In your example user.name field is just a string - the reflect package will have no way of correlating that back to the original struct.

Instead, you need to pass around the reflect.StructField for the given field:

field, ok := reflect.TypeOf(user).Elem().FieldByName("name")
…
tag = string(field.Tag)

Note: we use Elem above because user is a pointer to a struct.

You can play with an example here.

edited Jul 09 '20 at 14:30

Cirelli94

1,714
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answered May 07 '14 at 02:43

djd

4,988
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6

Your playground example had bugs, which I fixed for those who do not know how: https://play.golang.org/p/0SJT8I6gGR5 – MikeSchinkel Dec 03 '18 at 09:43
What if I don't want to pass the entire struct but just get the value of a specific field? reflect.TypeOf(user.Name).Elem(). – vlio20 Nov 22 '22 at 21:12

Raj Nandan Sharma · Answer 2 · 2021-10-29T07:31:02.290

Modifying the above answer can give value of a specific tag

package main

import (
    "fmt"
    "reflect"
)

type User struct {
    Name string `json:"name_field"`
    Age  int
}

func main() {
    user := &User{"John Doe The Fourth", 20}

    field, ok := reflect.TypeOf(user).Elem().FieldByName("Name")
    if !ok {
        panic("Field not found")
    }
    fmt.Println(getStructTag(field, "json")) //name_field
}

func getStructTag(f reflect.StructField, tagName string) string {
    return string(f.Tag.Get(tagName))
}

https://play.golang.org/p/Sb0i7za5Uow

score 2 · Answer 3 · answered Sep 16 '22 at 13:22

Clean way to list all tags from a struct (using external lib).

External lib: https://github.com/fatih/structs

Example: https://go.dev/play/p/C_yXMdbFYAq

package main

import (
    "fmt"

    "github.com/fatih/structs"
)

type User struct {
    Name string `json:"name_field"`
    Age  int    `json:"age_field"`
}

func main() {
    user := &User{}
    fields := structs.Fields(user)

    for _, field := range fields {
        tag := field.Tag("json")
        fmt.Println(tag)
    }
}

Result:

name_field
age_field

Get struct field tag using Go reflect package

3 Answers3

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