Golang的Tag语法

TagTag

结构

Structfieldfield

package main
import "fmt"
type T1 struct {
    f1 string
}
type T2 struct {
    T1
    f2     int64
    f3, f4 float64
}
func main() {
    t := T2{T1{"foo"}, 1, 2, 3}
    fmt.Println(t.f1)    // foo
    fmt.Println(t.T1.f1) // foo
    fmt.Println(t.f2)    // 1
}

fieldfieldFieldf3f4

标签

tag

type T struct {
    f1     string "f one"
    f2     string
    f3     string `f three`
    f4, f5 int64  `f four and five`
}

raw stringinterpreted stringtagfieldtag

反射

Tagreflection

package main
import (
    "fmt"
    "reflect"
)
type T struct {
    f1     string "f one"
    f2     string
    f3     string `f three`
    f4, f5 int64  `f four and five`
}
func main() {
    t := reflect.TypeOf(T{})
    f1, _ := t.FieldByName("f1")
    fmt.Println(f1.Tag) // f one
    f4, _ := t.FieldByName("f4")
    fmt.Println(f4.Tag) // f four and five
    f5, _ := t.FieldByName("f5")
    fmt.Println(f5.Tag) // f four and five
}

tagtag

type T struct {
    f1 string ``
    f2 string
}
func main() {
    t := reflect.TypeOf(T{})
    f1, _ := t.FieldByName("f1")
    fmt.Printf("%q\n", f1.Tag) // ""
    f2, _ := t.FieldByName("f2")
    fmt.Printf("%q\n", f2.Tag) // ""
}

格式

Tagskey1:"value1" key2:"value2" key3:"value3"LookupGetLookup

type T struct {
    f string `one:"1" two:"2"blank:""`
}
func main() {
    t := reflect.TypeOf(T{})
    f, _ := t.FieldByName("f")
    fmt.Println(f.Tag) // one:"1" two:"2"blank:""
    v, ok := f.Tag.Lookup("one")
    fmt.Printf("%s, %t\n", v, ok) // 1, true
    v, ok = f.Tag.Lookup("blank")
    fmt.Printf("%s, %t\n", v, ok) // , true
    v, ok = f.Tag.Lookup("five")
    fmt.Printf("%s, %t\n", v, ok) // , false
}

GetLookup

func (tag StructTag) Get(key string) string {
    v, _ := tag.Lookup(key)
    return v
}

转化

将结构体的值转化成其他的类型可通过Tag来定义

type T1 struct {
     f int `json:"foo"`
 }
 type T2 struct {
     f int `json:"bar"`
 }
 t1 := T1{10}
 var t2 T2
 t2 = T2(t1)
 fmt.Println(t2) // {10}

什么时候用到了Tag

(Un)marshaling

marshaling

import (
    "encoding/json"
    "fmt"
)
func main() {
    type T struct {
       F1 int `json:"f_1"`
       F2 int `json:"f_2,omitempty"`
       F3 int `json:"f_3,omitempty"`
       F4 int `json:"-"`
    }
    t := T{1, 0, 2, 3}
    b, err := json.Marshal(t)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%s\n", b) // {"f_1":1,"f_3":2}
}

ORM

比如说GORM— example.

go vet

tagstag

package main
type T struct {
    f string "one two three"
}
func main() {}
> go vet tags.go
tags.go:4: struct field tag `one two three` not compatible with reflect.StructTag.Get: bad syntax for struct tag pair