反映：设置指针的字段

Question

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 main

import (
    "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?

Answer 1

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