# struct tag

Struct是由一组field组成，每个field屬性包括了命名和類型，Field可以像f3和f4公用一个類型

### struct tag

* 有時候會在字段定義後面带上一个字符串(tag)，Tags可以由鍵值對组成，空格用來分割鍵值對
* <mark style="background-color:yellow;">Tag可以使用reflection包來讀取，並可以通過Lookup(建議)或者Get來獲取鍵值對的值</mark>。
* <mark style="background-color:blue;">什麼時候用到了Tag</mark>? ex json(Un)marshaling/ORM/go vet可以檢查出你的tag是否合理等等都有利用到tag來做解析。

```
import (
	"encoding/json"
	"fmt"
	"reflect"
)
type T struct {
	f1     string "f one"
	f2     string
	f3     string `f three 123`
	f4, f5 int64  `one:"1" two:"2"blank:""`
	Name   string `json:"name"` //注意要大寫開頭命名
	na     int    `json:"na"` //小寫開頭命名json會被丟棄
}
func main() {
	t := reflect.TypeOf(T{})
	f1, _ := t.FieldByName("f1")
	fmt.Println(f1.Tag) // f one
	f4, _ := t.FieldByName("f4")
	fmt.Println(f4.Tag) // one:"1" two:"2"blank:""
	f5, _ := t.FieldByName("f5")
	fmt.Println(f5.Tag) // one:"1" two:"2"blank:"" 共用 同上
	v, ok := f5.Tag.Lookup("one")
	fmt.Printf("%s, %t\n", v, ok) //1, true

	tt := T{"1", "0", "2", 3, 6, "kiki", 6}
	b, err := json.Marshal(tt)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s\n", b) //{"name":"kiki"} 只有json tag會被解析出來

	
}
```

#### json/bson/swagger 用到的範例

參考:<https://github.com/swaggo/swag>

```
type Who struct {
	Id        primitive.ObjectID `json:"id" bson:"_id,omitempty"` //mongo ObjectId
	Sub      `bson:",inline"`    // inline stored in mongo
	Docs string `json:"docs,omitempty"`
	Name       string   `json:"name" bson:"name" example:"mira"`
 	IsActive *bool `json:"isActive,omitempty"`
}
type Sub struct {
	Name       string   `json:"name" bson:"name" example:"name"`
	Labels      []string `json:"labels" bson:"labels" example:"123"`
	Description string   `json:"description"  bson:"description" example:"1234567"`
 Bar string `minLength:"4" maxLength:"16"`
 Baz int `minimum:"10" maximum:"20" default:"15"`
  Qux []string `enums:"foo,bar,baz"`
}
```

{% hint style="info" %}
Tip: 工作上其實常常會需要加上bson/json等等tag

一個個手動加太麻煩了，但有一個工具叫gomodifytags可以在vscode去使用喔！

&#x20;詳細用法請見：  <https://github.com/fatih/gomodifytags>
{% endhint %}

####

####

#### 參考及延伸文章:

Golang的Tag语法<https://studygolang.com/articles/14469>

自訂golang如何使用struct的tag屬性 //<https://www.gushiciku.cn/pl/2xWn/zh-tw>


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://minilabmemo.gitbook.io/golang-memo/basic/go-basic/struct/struct-tag.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.