What's the preferred way to encode a “nullable” field in protobuf 2?

Question

I am defining a ProtoBuf message where I want to have a \"nullable\" field -- i.e., I want to distinguish between the field having a value and not having a value. As a concrete

Answer 1

Have a notion of NaN for each of the types and then use default (as shown below) to set it as the value. This will be used if nothing is specified for that particular field.

optional float x = 1 [default = -1];