Iterate through the fields of a struct in Go

Question

Basically, the only way (that I know of) to iterate through the values of the fields of a struct is like this:

type Example struct {
    a_number uint32
    a_string string
}

//...

r := &Example{(2 << 31) - 1, "...."}:
for _, d:= range []interface{}{ r.a_number, r.a_string, } {
  //do something with the d
}

I was wondering, if there's a better and more versatile way of achieving []interface{}{ r.a_number, r.a_string, }, so I don't need to list each parameter individually, or alternatively, is there a better way to loop through a struct?

I tried to look through the reflect package, but I hit a wall, because I'm not sure what to do once I retrieve reflect.ValueOf(*r).Field(0).

Thanks!

Answer 1

After you've retrieved the reflect.Value of the field by using Field(i) you can get a interface value from it by calling Interface(). Said interface value then represents the value of the field.

There is no function to convert the value of the field to a concrete type as there are, as you may know, no generics in go. Thus, there is no function with the signature GetValue() T with T being the type of that field (which changes of course, depending on the field).

The closest you can achieve in go is GetValue() interface{} and this is exactly what reflect.Value.Interface() offers.

The following code illustrates how to get the values of each exported field in a struct using reflection (play):

import (
    "fmt"
    "reflect"
)

func main() {
    x := struct{Foo string; Bar int }{"foo", 2}

    v := reflect.ValueOf(x)

    values := make([]interface{}, v.NumField())

    for i := 0; i < v.NumField(); i++ {
        values[i] = v.Field(i).Interface()
    }

    fmt.Println(values)
}

Answer 2

If you want to Iterate through the Fields and Values of a struct then you can use the below Go code as a reference.

package main

import (
    "fmt"
    "reflect"
)

type Student struct {
    Fname  string
    Lname  string
    City   string
    Mobile int64
}

func main() {
    s := Student{"Chetan", "Kumar", "Bangalore", 7777777777}
    v := reflect.ValueOf(s)
    typeOfS := v.Type()

    for i := 0; i< v.NumField(); i++ {
        fmt.Printf("Field: %s\tValue: %v\n", typeOfS.Field(i).Name, v.Field(i).Interface())
    }
}

Run in playground

Note: If the Fields in your struct are not exported then the v.Field(i).Interface() will give panic panic: reflect.Value.Interface: cannot return value obtained from unexported field or method.