> Fields with scalar types are always present. Even if you don’t set them. Did I mention that (at least in proto31) all protobuffers can be zero-initialized with absolutely no data in them? Scalar fields get false-y values—uint32 is initialized to 0 for example, and string is initialized as "".

> It’s impossible to differentiate a field that was missing in a protobuffer from one that was assigned to the default value. Presumably this decision is in place in order to allow for an optimization of not needing to send default scalar values over the wire.

I believe there’s a trick you can do if you mark it as a “oneof” with only one field.