Setting values of nested struct fields using reflection in Go

Question

I'm trying to create a protobuf structure out of a

type FlatMessage map[string][]byte

and I'm struggling with setting values of nested fields.

I have an

func (fm *FlatMessage) Unflatten() (pb.Message, error)

method to transform flat message into structured protobuf.

I also have a helper function called analyze which takes a struct of type pb.Message and returns a map[int]*ProtoField where:

type ProtoField struct {
    Name  string
    Type  reflect.Type
}

ie analyze traverses pb.Message recursively and gathers all the information I need about it. So far so good.

Now when I go key by key through my FlatMessage , the key is an encoded field number of a respective protobuf field, and I can set it using reflection, like this:

r := reflect.ValueOf(&result).Elem().FieldByName(field.Name)
if r.IsValid() {
    r.Set(scalarValue)
}

but that works only when the field.Name in question does not refer to a nested field, ie, setting OldV1Id works fine, but attempting to set Profile.Id or, say, Destination.Address.TypeOfNumber , results in:

panic: reflect: call of reflect.Value.Set on zero Value [recovered]
    panic: reflect: call of reflect.Value.Set on zero Value

I understand that this is due to the fact that my r becomes <invalid Value> when called on a field which is a nested one, but how can I work around it?

My r is of course not valid , not addressable, and not settable. I can make a reflect.New out of it, but I can't figure out how to set that new reflect.Value in such a way that the field of my original structures becomes modified. No matter what I do the my function doesn't modify fields with nested names with this approach.

Another solution I tried is adding a Value reflect.Value to my ProtoField structure, and modifying analyze so that it appends a reflect.ValueOf(s).Field(i) where s is the top-level struct interface{} and i is its ith field. Then whenever I encounter a field that is a nested one, I call r := reflect.ValueOf(field.Value) , but then the problem is I'm unable to call r.Set(scalarValue) because of incompatible types.

Any help or insight is much appreciated.

Answer 1

Problem solved. ThunderCat was right in his comment: the trick is to use FieldByIndex . I didn't notice the signature of that method before, I thought it accepts an integer as an argument, but in fact it takes a slice of integers. It was then easy to modify my analyze function that traverses the structure recursively and now also constructs a slice of indexes as it goes. Now I can just r := reflect.ValueOf(&result).Elem().FieldByIndex(field.Idx) and I'm laughing.

Lesson learned here: if you want to set values of nested struct fields using reflection, don't call them by name; call them by index.