英文:
Reflect: setting a field of a pointer
问题
我正在尝试做类似这样的事情：
定义带有名为env的标签的结构体：
type Env struct {Port string `env:"PORT"`}
调用某个函数，该函数将使用os.Getenv获取环境变量名称，并将其设置到结构体中。
目前，我有以下代码：
package mainimport ("fmt""os""reflect")func ParseEnv(t interface{}, v interface{}) {it := reflect.TypeOf(t)for i := 0; i < it.NumField(); i++ {field := it.Field(i)value := os.Getenv(field.Tag.Get("env"))if value == "" {continue}reflect.ValueOf(v).Elem().FieldByName(field.Name).SetString(value)}}type Env struct {Port string `env:"PORT"`DatabaseURL string `env:"DATABASE_URL"`}func main() {os.Setenv("PORT", "8080")os.Setenv("DATABASE_URL", "postgres://user:pass@host:5432/my-db")env := Env{}ParseEnv(env, &env)fmt.Println(env)}
但是，正如你所看到的，我必须同时传递引用和指针给我的函数。
虽然这个方法可以工作，但是我觉得它很丑陋。
如果我只尝试传递指针，我无法得到正确的类型（因为它将是*interface{}），如果我只传递引用，我无法使用reflect设置值（即使我可以，它也不会起作用）。
有没有一种更合理的方法来做到这一点？
英文:
I'm trying to do something like this:
Define structs with tags named env:
type Env struct {Port string `env:"PORT"`}
Call some function which will get the environment variable names using os.Getenv and put set it in the struct.
Right now, I have this:
package mainimport ("fmt""os""reflect")func ParseEnv(t interface{}, v interface{}) {it := reflect.TypeOf(t)for i := 0; i < it.NumField(); i++ {field := it.Field(i)value := os.Getenv(field.Tag.Get("env"))if value == "" {continue}reflect.ValueOf(v).Elem().FieldByName(field.Name).SetString(value)}}type Env struct {Port string `env:"PORT"`DatabaseURL string `env:"DATABASE_URL"`}func main() {os.Setenv("PORT", "8080")os.Setenv("DATABASE_URL", "postgres://user:pass@host:5432/my-db")env := Env{}ParseEnv(env, &env)fmt.Println(env)}
http://play.golang.org/p/b8uPPVo4aV
But, as you can see, I have to pass both the reference and the pointer to my function.
While this works, it is very ugly (at least I think it is).
If I try to pass the pointer only, I can't get the type right (because it will be an *interface{}) and, if I pass only the reference, I can't set the values using reflect (even if I could, it would not work).
Is there a sane way of doing this?
答案1
得分: 4
以下是一种更合理的实现你想要的功能的方式。你会注意到，我们只需要传递一个指向结构体的指针，而不是传递两个结构体的副本。
func ParseEnv(val interface{}) {ptrRef := reflect.ValueOf(val)if ptrRef.Kind() != reflect.Ptr {panic("pointer to struct expected")}ref := ptrRef.Elem()if ref.Kind() != reflect.Struct {panic("pointer to struct expected")}refType := ref.Type()for i := 0; i < refType.NumField(); i++ {field := refType.Field(i)value := os.Getenv(field.Tag.Get("env"))if value == "" {continue}ref.Field(i).SetString(value)}}
以上函数应该以以下方式调用：
ParseEnv(&env)
示例：https://play.golang.org/p/_BwWz2oUql
英文:
Below is a "saner" way of doing what you want. You will notice that, instead of passing in two copies of the struct, we only need a pointer to the struct.
func ParseEnv(val interface{}) {ptrRef := reflect.ValueOf(val)if ptrRef.Kind() != reflect.Ptr {panic("pointer to struct expected")}ref := ptrRef.Elem()if ref.Kind() != reflect.Struct {panic("pointer to struct expected")}refType := ref.Type()for i := 0; i < refType.NumField(); i++ {field := refType.Field(i)value := os.Getenv(field.Tag.Get("env"))if value == "" {continue}ref.Field(i).SetString(value)}}
The above function should be invoked in the following way:
ParseEnv(&env)
Example: https://play.golang.org/p/_BwWz2oUql
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