Get struct Tag with reflection - Error: type reflect.Value has no field or method Tag

Question

假设我们有这个 PoC:

package main

import (
        "fmt"
        "reflect"
        "strings"
)

type MyStruct struct {
        Str string `gorm:"size:10" json:"string"`
}

func main() {
        aStruct := MyStruct{"Hello"}

        s := reflect.ValueOf(&aStruct).Elem()
        if s.Kind() == reflect.Struct {
                for i := 0; i < s.NumField(); i++ {
                        field := s.Field(i)
                        if field.IsValid() {
                                if field.CanSet() {
                                        fmt.Printf("%T\n", field)  // reflect.Value, need reflect.StructField
                                        gormTag := field.Tag.Get("gorm")  // Compile ERROR HERE
                                        gormSize := strings.Split(gormTag, "size:")[1]
                                        fmt.Println(gormSize)
                                }
                        }
                }
        }
}

错误是：

go run test1.go 
# command-line-arguments
./test1.go:22:22: field.Tag undefined (type reflect.Value has no field or method Tag)

使用 go 1.14.6 和 go 1.15.2 进行测试。

根据我的理解，我需要将 reflect.Value 转换（或从 reflect.Value 转换为 reflect.StructField 有什么想法吗？

Answer 1

标签属于结构字段的类型，不属于结构类型的值。

reflect.ValueOf() returns you the wrapper of the value, not its type. To get the tag value, you need to start from the wrapper of the type, e.g. acquired with reflect.TypeOf():

t := reflect.TypeOf(&aStruct).Elem()

以及循环中字段的类型包装器（它将是 reflect.StructTag 类型）：

field := s.Field(i)
fieldt := t.Field(i)

并使用fieldt获取标签：

gormTag := fieldt.Tag.Get("gorm") // No error, this works

添加后，输出将是（在 Go Playground 上尝试）：

reflect.Value
10

查看相关：What are the use(s) for tags in Go?

Get struct Tag with reflection - Error: type reflect.Value has no field or method Tag

Get struct Tag with reflection - Error: type reflect.Value has no field or method Tag

go

struct

reflection

introspection