Golang dynamic variable reference - variables

Question

In Go, I would like to do something like this. I have a big object with many structs (using Google's protobuf). here is a contrived example:
person.name = "testing"
person.address.street = "123 test st"
person.address.city = "tester"
person.address.zip = 90210
person.billing.address.same = true
I would like to be able to dynamically reference things. for example:
key := "person.address.zip"
fmt.Println("the value of key: " + key) // would like to get 90210
key := "person.address.city"
fmt.Println("the value of key: " + key) // would like to get "tester"
Is this possible in Go? if so, how could I do that? essentially, I'm creating a report which only contains a subset of the object and I want to be able to create a mapping file where the user can map keys/values together and my program will output the value. I have this working in python, but wanted to try using Go :)

Answer 1

You may use func (v Value) FieldByName(name string) Value from reflect package:
FieldByName returns the struct field with the given name. It returns
the zero Value if no field was found. It panics if v's Kind is not
struct.
Like this working sample code:
package main
import "fmt"
import "reflect"
func main() {
person := Person{}
person.name = "testing"
person.address.street = "123 test st"
person.address.city = "tester"
person.address.zip = 90210
person.billing.address.same = true
v := reflect.ValueOf(person)
f := v.FieldByName("address")
key := f.FieldByName("zip")
fmt.Println(key) // 90210
fmt.Println(f.FieldByName("city")) // tester
}
type Person struct {
name string
address Address
billing Billing
}
type Billing struct {
address Address
}
type Address struct {
street, city string
zip int
same bool
}
output:
90210
tester
And for your special case, you may use fmt.Println(field(person, "person.address.zip")), like this working sample code (just for demonstration):
package main
import "fmt"
import "reflect"
import "strings"
func field(t interface{}, key string) reflect.Value {
strs := strings.Split(key, ".")
v := reflect.ValueOf(t)
for _, s := range strs[1:] {
v = v.FieldByName(s)
}
return v
}
func main() {
person := Person{}
person.name = "testing"
person.address.street = "123 test st"
person.address.city = "tester"
person.address.zip = 90210
person.billing.address.same = true
fmt.Println(field(person, "person.address.zip")) //90210
fmt.Println(field(person, "person.address.city")) //tester
}
type Person struct {
name string
address Address
billing Billing
}
type Billing struct {
address Address
}
type Address struct {
street, city string
zip int
same bool
}
output:
90210
tester

Answer 2

I'm not familiar with protobuf's internals or if it provides any means to do that.
But, (1) if you like to read the value in the way you described - by chaining fields dynamically and (2) you want to read it more than one time; I would just serialize it into json and use this package. It's very fast and gives you (almost) the same semantic you desire:
// assuming your object got marshaled to this for example
json := `{"name":{"first":"Janet","last":"Prichard"},"age":47}`
value := gjson.Get(json, "name.last")
println(value.String())