Reflecting struct fields
- Go course
- Advanced
- Lesson 4/68
- 4 min
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")) // adPractice 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.
Starter code
package main
import (
"fmt"
"reflect"
)
func main() {
// buraya yaz
}Sample 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))
}
}Expected output
Ad Ece Not 90
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- Field(i).Int() on the reflect.Value
Explanation: Field name, tag and type belong to the type (StructField); the value side only carries the contents.
2 Field count
for i := 0; i < t.____(); i++ {for i := 0; i < t.NumField(); i++ {Explanation: NumField() returns the number of fields in a struct type.
3 Struct reflect parts
| Term | Meaning |
|---|---|
NumField() |
Number of fields |
Field(i).Name |
Field name |
Tag.Get("json") |
Tag value |
FieldByName |
Finds a field by name |