Go struct tags
last modified April 11, 2024
In this article we show how to work with struct tags in Golang.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
A struct is a user-defined type that contains a collection of fields. It is used to group related data to form a single unit. A Go struct can be compared to a lightweight class without the inheritance feature.
A struct tag is additional meta data information inserted into struct fields. The meta data can be acquired through reflection. Struct tags usually provide instructions on how a struct field is encoded to or decoded from a format.
Struct tags are used in popular packages including:
- encoding/json
- encoding/xml
- gopkg.in/mgo.v2/bson
- gorm.io/gorm
- github.com/gocarina/gocsv
- gopkg.in/yaml.v2
Go struct tag json
In the next example, we use the json struct tag with the
encoding/json package.
package main
import (
"encoding/json"
"fmt"
)
type User struct {
Id int `json:"id"`
Name string `json:"name"`
Occupation string `json:"occupation,omitempty"`
}
func (p User) String() string {
return fmt.Sprintf("User id=%v, name=%v, occupation=%v",
p.Id, p.Name, p.Occupation)
}
func main() {
user := User{Id: 1, Name: "John Doe", Occupation: "gardener"}
res, _ := json.MarshalIndent(user, " ", " ")
fmt.Println(string(res))
user2 := User{Id: 1, Name: "John Doe"}
res2, _ := json.MarshalIndent(user2, " ", " ")
fmt.Println(string(res2))
}
The example uses struct tags to configure how JSON data is encoded.
type User struct {
Id int `json:"id"`
Name string `json:"name"`
Occupation string `json:"occupation,omitempty"`
}
With `json:"id"` struct tag, we encode the Id field in lowercase.
In addition, the omitempty omits the Occupation field
if it is empty.
$ go run main.go
{
"id": 1,
"name": "John Doe",
"occupation": "gardener"
}
{
"id": 1,
"name": "John Doe"
}
Go struct tag xml
In the following example, we use the xml struct tag with the
encoding/xml package.
package main
import (
"encoding/xml"
"fmt"
)
type User struct {
Id int `xml:"id"`
Name string `xml:"name"`
Occupation string `xml:"occupation"`
}
func (p User) String() string {
return fmt.Sprintf("User id=%v, name=%v, occupation=%v",
p.Id, p.Name, p.Occupation)
}
func main() {
user := User{Id: 1, Name: "John Doe", Occupation: "gardener"}
res, _ := xml.MarshalIndent(user, " ", " ")
fmt.Println(xml.Header + string(res))
}
The example turns a Go structure into XML format. Using struct tags we can configure the output.
$ go run main.go <?xml version="1.0" encoding="UTF-8"?> <User> <id>1</id> <name>John Doe</name> <occupation>gardener</occupation> </User>
Go struct tag csv
The next example uses the csv struct tag with the
github.com/gocarina/gocsv library. The package provides easy
serialization and deserialization functions to use CSV in Golang.
package main
import (
"fmt"
"github.com/gocarina/gocsv"
)
type User struct {
Id string `csv:"user_id"`
Name string `csv:"user_name"`
Occupation string `csv:"user_occupation"`
}
func (p User) String() string {
return fmt.Sprintf("User id=%v, name=%v, occupation=%v",
p.Id, p.Name, p.Occupation)
}
func main() {
users := []User{}
users = append(users, User{Id: "1", Name: "John Doe", Occupation: "gardener"})
users = append(users, User{Id: "2", Name: "Roger Doe", Occupation: "driver"})
res, _ := gocsv.MarshalString(users)
fmt.Println(res)
}
We transform a slice of user structures into CSV; the struct tags configure the header names.
$ go run main.go user_id,user_name,user_occupation 1,John Doe,gardener 2,Roger Doe,driver
Source
The Go Programming Language Specification
In this article we have covered struct tags in Golang.
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