Update the fields of one struct to another struct - go

Question

I have a struct:
type Person struct {
FirstName string
LastName int
Age int
HairColor string
EyeColor string
Height string
}
And I have a function that takes in 2 parameters, and updates the first Person's fields to the second person's fields:
func updateFields(personA *Person, personB Person) {
personA.FirstName = personB.FirstName
personA.LastName = personB.LastName
// Don't want to change Age.
personA.HairColor = personB.HairColor
personA.EyeColor = personB.EyeColor
personA.Height = personB.Height
}
Instead of hard-coding the values to change, how can I loop through the fields and update the first Person to have the same values as the second Person except for the "Age" field?

Answer 1

You can just copy the entire personB into personA and make a backup of the Age field before:
func updateFields(personA *Person, personB Person) {
tempAge := personA.Age
*personA = personB
personA.Age = tempAge
}

Answer 2

For trivially copying every field, you can simply do something like *personA = personB. If you need to "not copy" just one specific field (same field every time), you could probably just save that field's value in a separate variable, copy everything with *personA = personB, and then copy the value back. But this is only useful for very specific situations. It wouldn't allow for example to have a dynamic set of fields not to copy.
If you want to do it with more flexibility, you can do it using reflection, sample code below.
Notice that there may be a few limitations. Notably, you can only set exported fields. Also, if you don't test for these limitations and accidentally try to set a field that's not settable, or with a value of type that's not assignable to that field, etc, the reflect package will happily panic. So it's wise to add a lot of checks before you actually .Set(...) the field.
import (
"fmt"
"reflect"
)
type Person struct {
FirstName string
LastName int
Age int
HairColor string
EyeColor string
Height string
}
func updateFields(personA *Person, personB Person) {
// .Elem() called to dereference the pointer
aVal := reflect.ValueOf(personA).Elem()
aTyp := aVal.Type()
// no .Elem() called here because it's not a pointer
bVal := reflect.ValueOf(personB)
for i := 0; i < aVal.NumField(); i++ {
// skip the "Age" field:
if aTyp.Field(i).Name == "Age" {
continue
}
// you might want to add some checks here,
// eg stuff like .CanSet(), to avoid panics
aVal.Field(i).Set(bVal.Field(i))
}
}
func main() {
b := Person{
FirstName: "Bruno",
LastName: 1,
Age: 2,
HairColor: "hello",
EyeColor: "world",
Height: "tall",
}
a := Person{}
fmt.Println(a)
updateFields(&a, b)
fmt.Println(a)
}