extension reflection question

[kmatzen]

I have two message types:

message Record {
  optional string foo = 1;
  optional int32 bar = 2;

  extensions 100 to max;
}

message SpecializedRecord {
  extend Record {
    optional SpecializedRecord parent = 100;
  }

  optional uint32 special_thing = 1;
}

I'm trying to use reflection to access special_thing by name in C++. First I retrieve parent and then get the extension message to do reflection on that. I'm having trouble getting the extension from the original message.

This works:

const Reflection* record_reflection = record.GetReflection();
vector<const FieldDescriptor*> fields;
record_reflection->ListFields(record, &fields);
const FieldDescriptor* extension_field = 0;
for (int i = 0; i < fields.size(); ++i) {
  if (fields.at(i)->name() == "parent") {
    extension_field = fields.at(i);
  }   
}
CHECK_NOTNULL(extension_field);

This doesn't work:

const Descriptor* record_descriptor = record.GetDescriptor();
const FieldDescriptor* extension_field = record_descriptor->FindFieldByName("parent");
CHECK_NOTNULL(extension_field);

Why?

[xfxyjwf]

FindFieldByName() only works for regular fields. For extensions, you need to call FindExtensionByName().

[kmatzen]

This also doesn't work:

const Descriptor* record_descriptor = record.GetDescriptor();
const FieldDescriptor* extension_field = record_descriptor->FindExtensionByName("parent");
CHECK_NOTNULL(extension_field);

Or did you not mean to use FindExtensionByName in this way?

[xfxyjwf]

Try this:

const DescriptorPool* pool = record.GetDescriptor()->file()->pool();
const FieldDescriptor* extension_field = pool->FindExtensionByName("SpecializedRecord.parent");

Note that the extension name must be fully qualified.