Get struct field tag using Go reflect package - reflection

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.

Answer 1

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.

Answer 2

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

Answer 3

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