Golang动态设置结构体字段默认值实践

type Config struct {
    Host string
    Port int
    SSL  bool
}

func NewConfig() *Config {
    return &Config{
        Host: "localhost",
        Port: 8080,
        SSL:  false,
    }
}

cfg := NewConfig()
cfg.Host = "example.com"

type User struct {
    Name string `default:"Anonymous"`
    Age  int    `default:"18"`
    Role string `default:"user"`
}

import (
    "reflect"
)

func SetDefaults(v interface{}) {
    rv := reflect.ValueOf(v)
    if rv.Kind() != reflect.Ptr || rv.IsNil() {
        return
    }
    elem := rv.Elem()
    for i := 0; i < elem.NumField(); i++ {
        field := elem.Field(i)
        if !field.CanSet() || !field.Interface().(== reflect.Zero(field.Type()).Interface()) {
            continue
        }
        tag := elem.Type().Field(i).Tag.Get("default")
        if tag == "" {
            continue
        }
        switch field.Kind() {
        case reflect.String:
            field.SetString(tag)
        case reflect.Int, reflect.Int32, reflect.Int64:
            if val, err := strconv.Atoi(tag); err == nil {
                field.SetInt(int64(val))
            }
        case reflect.Bool:
            if val, err := strconv.ParseBool(tag); err == nil {
                field.SetBool(val)
            }
        }
    }
}

user := &User{Name: "", Age: 0}
SetDefaults(user)
// user.Name == "Anonymous", user.Age == 18, user.Role == "user"

type ServerConfig struct {
    Host string `mapstructure:"host" default:"0.0.0.0"`
    Port int    `mapstructure:"port" default:"9000"`
}