Go
Struct tags
Learn to control field names with json:"..." tags.
Struct tags
JSON field names are usually lower case. A struct tag maps the Go field name to a JSON name and adds options:
type Kullanici struct {
ID int `json:"id"`
Ad string `json:"ad"`
Eposta string `json:"eposta,omitempty"`
Sifre string `json:"-"`
}
omitempty drops a zero-valued field from the output; - always skips the field (for secrets such as passwords).
Tag syntax
Tags are key:"value" pairs inside backticks; several libraries can use their own key on the same field:
Ad string `json:"ad" db:"ad_soyad" validate:"required"`
When decoding (Unmarshal), JSON keys are matched to tags case-insensitively. Unknown keys are silently ignored; to reject them use Decoder.DisallowUnknownFields().
Exercise Tagged encoding
Write type User struct: ID int → "id", Name string → "name", Email string → "email,omitempty", Password string → "-". Encode User{ID: 1, Name: "Ada", Password: "x"} and print it.
Show example solution
package main
import (
"encoding/json"
"fmt"
)
type User struct {
ID int `json:"id"`
Name string `json:"name"`
Email string `json:"email,omitempty"`
Password string `json:"-"`
}
func main() {
b, err := json.Marshal(User{ID: 1, Name: "Ada", Password: "x"})
if err != nil {
fmt.Println(err)
return
}
fmt.Println(string(b))
}
Test yourself
-
If an <code>int</code> field tagged <code>`json:"adet,omitempty"`</code> is 0, what happens?
- "adet":null is written
- Marshal returns an error
- The field isn't written to the JSON at all
- "adet":0 is written
Show answer
The field isn't written to the JSON at all — omitempty drops zero values (0, "", false, nil, empty slice/map); use it carefully where 0 is meaningful, or prefer *int.
-
Hidden field
type Hesap struct {
Ad string `json:"ad"`
Token string `json:"____"`
}
Show answer
type Hesap struct {
Ad string `json:"ad"`
Token string `json:"-"`
}
-
Tag and effect
- json:"id"
- Renames the field
- json:",omitempty"
- Skips if zero
- json:"-"
- Always skips
- json:",string"
- Writes a number as a string
Next lesson: Marshal and Unmarshal
