golang - reflection on embedded structs - go

Question

Given a struct like so:
type B struct {
X string
Y string
}
type D struct {
B
Z string
}
I want to reflect on D and get to the fields X, Y, Z.
Intuitively, before attempting the solution, I was assuming I would be able to traverse the struct D and get all fields using reflection (X, Y, Z) and won't have to deal with B.
But as you can see, I only see the embedded struct B using reflection and not its fields.
http://play.golang.org/p/qZQD5GdTA8
Is there a way I can make B fully transparent when reflecting on D?
Why do I want this?
Imaging a common struct (B in the example here), that is used in multiple other structs by using embedding. Using reflection, the attempt is to copy D into another similar struct in a different package. The destination struct for copying will have all attributes flatly laid out (no embedding there). So there is a mismatch from the source to the destination (embedding vs no embedding) but all the attributes flatly laid out are the same. I don't want to create custom solutions for each struct.

Answer 1

The 'transparency' you expected is just syntactic sugar and has nothing to do with the data representation. If you want to have a function that flattens your data structure, you would have to write it by yourself.
For example (On play):
func DeepFields(iface interface{}) []reflect.Value {
fields := make([]reflect.Value, 0)
ifv := reflect.ValueOf(iface)
ift := reflect.TypeOf(iface)
for i := 0; i < ift.NumField(); i++ {
v := ifv.Field(i)
switch v.Kind() {
case reflect.Struct:
fields = append(fields, DeepFields(v.Interface())...)
default:
fields = append(fields, v)
}
}
return fields
}

Answer 2

Use the following code to collect all promoted field names as keys in map m:
func collectFieldNames(t reflect.Type, m map[string]struct{}) {
// Return if not struct or pointer to struct.
if t.Kind() == reflect.Ptr {
t = t.Elem()
}
if t.Kind() != reflect.Struct {
return
}
// Iterate through fields collecting names in map.
for i := 0; i < t.NumField(); i++ {
sf := t.Field(i)
m[sf.Name] = struct{}{}
// Recurse into anonymous fields.
if sf.Anonymous {
collectFieldNames(sf.Type, m)
}
}
}
Use it like this:
m := make(map[string]struct{})
collectFieldNames(reflect.TypeOf((*D)(nil)), m)
for name := range m {
fmt.Println(name)
}
Run it on the playground.
This program prints X, Y an Z as requested in the question, but also B because B is also a field name.
This function in this answer can be improved:
Don't blow up on recursive type definitions.
Do not include names repeated at the same level in the hierarchy.
The typeField function in encoding/json/encode.go handles both of these issues.