Golang : What is StructTag and how to get StructTag's value?

For those who code or read Golang's code and left wondering what are those string inside the '``' such as xml:"firstname" Such as these example below :

 type Staff struct {
 XMLName xml.Name `xml:"staff"`
 ID int `xml:"id"`
 FirstName string `xml:"firstname"`
 LastName string `xml:"lastname"`
 UserName string `xml:"username"`
 }

 type Company struct {
 XMLName xml.Name `xml:"company"`
 Staffs []Staff  `xml:"staff"`
 }

They are known as StructTag and you can process the tags with the reflect package. In this example, we will extract the custom tags' value with field.Tag.Get() method :

 package main

 import (
  "encoding/xml"
  "fmt"
  "reflect"
 )

 func main() {

  type Company struct {
 XMLName xml.Name `xml:"company"`
  }

  c := Company{}
  cType := reflect.TypeOf(c)
  field := cType.Field(0)

  xmlTag := field.Tag.Get("xml")
  fmt.Println(xmlTag)

  type S struct {
 F string `species:"gopher" color:"pink"`
  }

  s := S{}
  st := reflect.TypeOf(s)
  field = st.Field(0)
  fmt.Println(field.Tag.Get("color"), field.Tag.Get("species"))

 }

Output :

company

pink gopher

One usage example that I can think of is that StructTag allows a developer to create her/his own custom tagging and use the tags to find the relevant field and extract value from the field.

Hope this short tutorials can be useful to you!

Reference :

http://golang.org/pkg/reflect/#StructTag