Get name of struct field using reflection

Question

What is the way of printing "Foo" here? In this example, what prints is "string".

http://play.golang.org/p/ZnK6PRwEPp

type A struct {
    Foo string
}

func (a *A) PrintFoo() {
    fmt.Println("Foo value is " + a.Foo)
}

func main() {
    a := &A{Foo: "afoo"}
    val := reflect.Indirect(reflect.ValueOf(a))
    fmt.Println(val.Field(0).Type().Name())
}

Answer 1

You want val.Type().Field(0).Name. The Field method on reflect.Type will return a struct describing that field, which includes the name, among other information.

There is no way to retrieve the field name for a reflect.Value representing a particular field value, since that is a property of the containing struct.

Answer 2

I think the better way to get the fields' name in the struct is

func main() {
    a := &A{Foo: "afoo"}
    val := reflect.ValueOf(a).Elem()
    for i:=0; i<val.NumField();i++{
        fmt.Println(val.Type().Field(i).Name)
    }
}

There are two tips:

1. use .Elem() after you reflect.ValueOf(a), because in your case, a is a pointer.
2. val.Field(i).Type().Name is totally different from val.Type().Field(i).Name. The latter one can get the name of the field in the struct

Hope that it is helpful..

If you want to have a look at more cases, please check my 2mins article

Answer 3

You need to Get the Field of the Type Definition not of the Value.

http://play.golang.org/p/7Bc7MJikbJ

package main

import "fmt"
import "reflect"

type A struct {
    Foo string
}

func (a *A) PrintFoo() {
    fmt.Println("Foo value is " + a.Foo)
}

func main() {
    a := &A{Foo: "afoo"}
    val := reflect.Indirect(reflect.ValueOf(a))
    fmt.Println(val.Type().Field(0).Name)
}

Answer 4

Note that this solution references a Go module which is now deprecated, and is no longer being maintained. It was deprecated as of Oct 11, 2018.

With the new Names method of the structs package it's even easier:

package main

import (
    "fmt"

    "github.com/fatih/structs"
)

type A struct {
    Foo string
    Bar int
}

func main() {
    names := structs.Names(&A{})
    fmt.Println(names) // ["Foo", "Bar"]
}

Answer 5

package main

import "fmt"
import "reflect"

type A struct {
    Foo string
}

func (a *A) PrintFoo() {
    fmt.Println("Foo value is " + a.Foo)
}

func main() {
    a := &A{Foo: "afoo"}

    //long and bored code
    t := reflect.TypeOf(*a)
    if t.Kind() == reflect.Struct {
        for i := 0; i < t.NumField(); i++ {
            fmt.Println(t.Field(i).Name)
        }
    } else {
        fmt.Println("not a stuct")
    }

    //shorthanded call
    fmt.Println(reflect.TypeOf(*a).Field(0).Name)//can panic if no field exists

}

Answer 6

You can use this function, which takes the struct as the first parameter, and then its fields. It returns the map type, which is convenient to use

If you use fields from another structure, nothing will happen

If you try to use a different type, it will cause panic

Note that the field has an ordinal number according to the list (starting from 0). All fields in the structure must start with uppercase

func GetStructFieldName(Struct interface{}, StructField ...interface{}) (fields map[int]string) {
    fields = make(map[int]string)
    s := reflect.ValueOf(Struct).Elem()

    for r := range StructField {
        f := reflect.ValueOf(StructField[r]).Elem()

        for i := 0; i < s.NumField(); i++ {
            valueField := s.Field(i)
            if valueField.Addr().Interface() == f.Addr().Interface() {
                fields[i] = s.Type().Field(i).Name
            }
        }
    }
    return fields
}

Full example and playground

package main

import (
    "fmt"
    "reflect"
)

type Example struct {
    Apple bool
    Pear  int
}

func GetStructFieldName(Struct interface{}, StructField ...interface{}) (fields map[int]string) {
    fields = make(map[int]string)

    for r := range StructField {
        s := reflect.ValueOf(Struct).Elem()
        f := reflect.ValueOf(StructField[r]).Elem()

        for i := 0; i < s.NumField(); i++ {
            valueField := s.Field(i)
            if valueField.Addr().Interface() == f.Addr().Interface() {
                fields[i] = s.Type().Field(i).Name
            }
        }
    }
    return fields
}

func main() {
    e := Example{}

    names := GetStructFieldName(&e, &e.Apple, &e.Pear)

    fmt.Println(names)
    fmt.Println(names[0], names[1])

    for i := range names {
        fmt.Println(names[i])
    }

    /* Output:
    map[0:Apple 1:Pear]
    Apple Pear
    Apple
    Pear
    */
}

Answer 7

You can also use https://github.com/fatih/structs

// Convert the fields of a struct to a []*Field
fields := s.Fields()

for _, f := range fields {
    fmt.Printf("field name: %+v\n", f.Name())
}