Go Struct 默认值的实现

当加载配置的时候，我们通常会将配置设置一个默认值，但是通常默认值不是Go中的空值，比如服务的端口号：

type ServerConfig struct{
    Port uint
}

func main(){
    conf:=loadConfig()

    if conf.ServerConfig.Port==0{
        conf.ServerConfig.Port = 8080
    }
}

每次有一个值我们都需要增加一个if语句进行判断，当一个配置有很多的时候，初始化写起来可能就非常的繁琐。

幸运的是Go中提供了tag，我们可以借用tag，结合反射来实现默认值。

原理比较简单大概流程就是：

通过反射解析当前的结构体
查看当前字段的值时候为空值
如果为空，读取tag中的default，并初始化
不为空直接略过

具体代码可以参考：https://github.com/zzjcool/goutils/blob/main/defaults/defauls.go

这里给出使用方法：

Defaults

Enforce default values on struct fields.

type User struct {
 Name     string  `default:"Goku"`
 Power    float64 `default:"9000.01"`
}

var u User

err := defaults.Apply(&u)
if err != nil {
 log.Fatal("Uh oh: %v", err)
}

fmt.Print(u.Name)  // Goku
fmt.Print(u.Power) // 9000.01

Defaults are only applied to fields at their zero value.

type Config struct {
 Host  *string `default:"0.0.0.0"`
 Port  *int    `default:"8000"`
}

var cfg Config
json.Unmarshal([]byte(`{Host: "charm.sh"}`), &cfg)

if err := defaults.Apply(&cfg); err != nil {
 log.Fatal("Rats: %v", err)
}

fmt.Print(cfg.Host) // charm.sh
fmt.Print(cfg.Port) // 8000

Works well with JSON, Yaml, and so on.

type Config struct {
    Host  string `json:"host" default:"0.0.0.0"`
    Port  int    `json:"port" default:"8000"`
    Debug bool   `json:"debug" default:"true"`
}

Supported Types

The following types are supported:

string
bool
int
int8
int16
int32 (and rune, with some caveats)
int64
uint
uint8
uint16
uint32
uint64
float32
float64
[]byte/[]uint8

…as well as pointers to those types.

Embedded Structs

Embedded structs are supported. The following will parse as expected:

type GroceryList struct {
 Fruit struct {
  Bananas int `default:"8"`
  Pears   int `default:"12"`
 }
 Vegetables *struct {
  Artichokes    int `default:"4"`
  SweetPotatoes int `default:"16"`
 }
}

Embedded structs do not need a default tag in order to be parsed. Embedded structs that are nil will be initialized with their zero value so they can be parsed accoringly.

Runes and Int32s

In Go rune is just an alias for int32. This presents some ambiguity when parsing default values. For example, should "1" be parsed as a literal 1 or as a unicode '1' (which has the int32 value of 49)?

Because of this ambiguity we recommend avoiding setting defaults on runes. That said, this package defaults to parsing int32 as integers. Failing that, it tries to parse them as a rune.

// This works as expected...
type Cool struct {
 Fave32BitInteger int32 `default:"12"`
 FaveChar         rune  `default:"a"`
}

// ...but these will not.
type UhOh struct {
 FaveChar rune `default:"3"`  // this is a unicode ETX or ctrl+c
 FaveChar rune `default:"97"` // this is a unicode `a`
}

这个package本来是： github.com/charmbracelet/defaults 但是原地址无法访问