Go struct tags

last modified April 11, 2024

In this article we show how to work with struct tags in Golang.

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.

main.go

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.

main.go

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.

main.go

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.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

List all Go tutorials.