Reflecting struct fields

Learn to access struct fields with reflect.

Iterating struct fields

NumField() gives the number of fields in a struct type; you reach each one with Field(i). The type gives the field name, the value gives its contents.

k := Kitap{Ad: "Dune", Sayfa: 412}
t, v := reflect.TypeOf(k), reflect.ValueOf(k)
for i := 0; i < t.NumField(); i++ {
    fmt.Println(t.Field(i).Name, v.Field(i))
}

Output: Ad Dune and Sayfa 412.

Struct tags and exporting

t.Field(i).Tag.Get("json") reads a field's tag; the JSON package finds field names this way. Unexported fields (lower-case names) can be read but not taken out with Interface() or changed.

type K struct {
    Ad string `json:"ad"`
}
f, _ := reflect.TypeOf(K{}).FieldByName("Ad")
fmt.Println(f.Tag.Get("json"))  // ad

Exercise List the fields

Define type Student struct { Ad string; Not int }. For Student{"Ece", 90}, loop over all fields with NumField and print each field name and value on its own line.

Show example solution

package main

import (
    "fmt"
    "reflect"
)

type Student struct {
    Ad  string
    Not int
}

func main() {
    o := Student{"Ece", 90}
    t, v := reflect.TypeOf(o), reflect.ValueOf(o)
    for i := 0; i < t.NumField(); i++ {
        fmt.Println(t.Field(i).Name, v.Field(i))
    }
}

Test yourself

Where do you read a field's <strong>name</strong> from?
- From the result of NumField()
- With fmt.Sprint
- Field(i).Name on the reflect.Type
- Field(i).Int() on the reflect.Value
Show answer

Field(i).Name on the reflect.Type — Field name, tag and type belong to the type (StructField); the value side only carries the contents.

Field count

for i := 0; i < t.____(); i++ {

Show answer

for i := 0; i < t.NumField(); i++ {

Struct reflect parts

NumField()

Number of fields
Field(i).Name

Field name
Tag.Get("json")

Tag value
FieldByName

Finds a field by name

Next lesson: Dynamic call

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies