Golang反射获取字段标签默认值实现

type User struct {
    Name  string `json:"name" default:"guest"`
    Age   int    `json:"age" default:"18"`
    Email string `json:"email"`
}

func SetDefaults(v interface{}) {
    rv := reflect.ValueOf(v)
    if rv.Kind() != reflect.Ptr || rv.IsNil() {
        return
    }
    rv = rv.Elem() // 解引用指针
    rt := rv.Type()

    for i := 0; i < rv.NumField(); i++ {
        field := rv.Field(i)
        if !field.CanSet() {
            continue
        }

        tag := rt.Field(i).Tag.Get("default")
        if tag == "" {
            continue
        }

        // 判断字段是否为零值
        if field.Interface() == reflect.Zero(field.Type()).Interface() {
            switch field.Kind() {
            case reflect.String:
                field.SetString(tag)
            case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
                if val, err := strconv.ParseInt(tag, 10, 64); err == nil {
                    field.SetInt(val)
                }
            case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
                if val, err := strconv.ParseUint(tag, 10, 64); err == nil {
                    field.SetUint(val)
                }
            case reflect.Bool:
                if val, err := strconv.ParseBool(tag); err == nil {
                    field.SetBool(val)
                }
            }
        }
    }
}

func main() {
    user := User{Name: "", Age: 0}
    SetDefaults(&user)
    fmt.Printf("%+v\n", user) // 输出: {Name:guest Age:18 Email:}
}