Struct Tags

Struct tags are small pieces of metadata attached to fields of a struct that provide instructions to other Go code that works with the struct.

Basic Usage

• Go struct tags are annotations that appear after the type in a Go struct declaration.
• Each tag is composed of short strings associated with some corresponding value.
• A struct tag looks like this, with the tag offset with backtick ` characters.

type User struct {
    Name string `example:"name"`
}

• Other Go code is then capable of examining these structs and extracting the values assigned to specific keys it requests.
• Struct tags have no effect on the operation of your code without additional code that examines them.

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Encoding json

• Struct tags can be used to encode JSON.
• The json tag is used to specify the name of the field when encoding to JSON.
• It can also be used to denote the field as private, which will exclude it from encoding.
• Or, omit this field in json if the value is empty.

package main

import (
    "encoding/json"
    "fmt"
    "log"
    "os"
    "time"
)

type User struct {
    Name          string    `json:"name"`
    Password      string    `json:"-"` // ignore private field
    PreferredFish []string  `json:"preferredFish, omitempty"` // omit if empty
    CreatedAt     time.Time `json:"createdAt"`
}

func main() {
    u := &User{
        Name:      "Sammy the Shark",
        Password:  "fisharegreat",
        CreatedAt: time.Now(),
    }

    out, err := json.MarshalIndent(u, "", "  ")
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }

    fmt.Println(string(out))
}