Reflection Basics

Struct Tags

Struct tags attach metadata that libraries can read through reflection.

struct tags Tags are strings attached to fields. Reflection can read them so encoders and validators can follow naming rules.

Struct Tags

fieldIndex
struct_tags_reflect.go
Replay: real traced execution (multi-file project)
package main

import (
	"fmt"
	"reflect"
)

type Item struct {
	SKU   string `json:"sku"`
	Count int    `json:"count"`
}

func main() {
	var fieldIndex = 0
	itemType := reflect.TypeOf(Item{})
	field := itemType.Field(fieldIndex)
	jsonName := field.Tag.Get("json")

	fmt.Println("field_index=", fieldIndex)
	fmt.Println("field_name=", field.Name)
	fmt.Println("field_type=", field.Type.String())
	fmt.Println("json_name=", jsonName)
}

package main

import (
	"fmt"
	"reflect"
)

type Item struct {
	SKU   string `json:"sku"`
	Count int    `json:"count"`
}

func main() {
	var fieldIndex = 1
	itemType := reflect.TypeOf(Item{})
	field := itemType.Field(fieldIndex)
	jsonName := field.Tag.Get("json")

	fmt.Println("field_index=", fieldIndex)
	fmt.Println("field_name=", field.Name)
	fmt.Println("field_type=", field.Type.String())
	fmt.Println("json_name=", jsonName)
}

  1. fieldIndex ← 0, itemType ← &reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}}

    13func main() {14  var fieldIndex→ 0 = 0 //@fieldIndex=115  itemType→ &reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}} := reflect.TypeOf(Item{})16  field→ reflect.StructField{Name:"SKU", PkgPath:"", Type:(*reflect.rtype)(⟨addr D⟩), Tag:"json:\"sku\"", Offset:0x0, Index:[]int{0}, Anonymous:false} := itemType&reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}}.Field(fieldIndex0)17  jsonName→ "sku" := field.Tag"json:\"sku\"".Get("json")1819  fmt.Println("field_index=", fieldIndex0)20  fmt.Println("field_name=", field.Name"SKU")21  fmt.Println("field_type=", field.Type&reflect.rtype{t:abi.Type{Size_:0x10, PtrBytes:0x8, Hash:⟨addr E⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x18, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr F⟩), GCData:(*uint8)(⟨addr C⟩), Str:3757, PtrToThis:20064}}.String())22  fmt.Println("json_name=", jsonName"sku")23}
    outputfield_index= 0
field_name= SKU
field_type= string
json_name= sku

  1. fieldIndex ← 1, itemType ← &reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}}

    13func main() {14  var fieldIndex→ 1 = 115  itemType→ &reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}} := reflect.TypeOf(Item{})16  field→ reflect.StructField{Name:"Count", PkgPath:"", Type:(*reflect.rtype)(⟨addr D⟩), Tag:"json:\"count\"", Offset:0x10, Index:[]int{1}, Anonymous:false} := itemType&reflect.rtype{t:abi.Type{Size_:0x18, PtrBytes:0x8, Hash:⟨addr A⟩, TFlag:0x7, Align_:0x8, FieldAlign_:0x8, Kind_:0x19, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr B⟩), GCData:(*uint8)(⟨addr C⟩), Str:6798, PtrToThis:19168}}.Field(fieldIndex1)17  jsonName→ "count" := field.Tag"json:\"count\"".Get("json")1819  fmt.Println("field_index=", fieldIndex1)20  fmt.Println("field_name=", field.Name"Count")21  fmt.Println("field_type=", field.Type&reflect.rtype{t:abi.Type{Size_:0x8, PtrBytes:0x0, Hash:⟨addr E⟩, TFlag:0xf, Align_:0x8, FieldAlign_:0x8, Kind_:0x2, Equal:(func(unsafe.Pointer, unsafe.Pointer) bool)(⟨addr F⟩), GCData:(*uint8)(⟨addr G⟩), Str:814, PtrToThis:20640}}.String())22  fmt.Println("json_name=", jsonName"count")23}
    outputfield_index= 1
field_name= Count
field_type= int
json_name= count