protobuf MessageToJson removes fields with value 0

There is no correct way, I'm defining my protobufs incorrectly. For enumerated fields, the first value is the default value. This means if a protobuf comes through without a set value, it is set to the default value and, when converted to json, ignored (unless you want to keep all default values.)

Thus, it is recommended use throw-away names for the default value to be able to properly distinguish when it has been set. i.e. I should define my protobuf as:

enum C_TYPE
{
    NONE = 0;
    AB = 1;
    BC = 2;
    CD = 3;
}

From Protobuf Documentation on Optional Fields And Default Values:

For enums, the default value is the first value listed in the enum's type definition. This means care must be taken when adding a value to the beginning of an enum value list.

Additionally from an issue on golang/protobuf:

This is working as intended. proto3 zero-values are omitted in the JSON format too. The zero-value should be a "throwaway" value: it's also what you will see if the sender of a serialized message sets the field to an invalid or unrecognized value.

In protobuf in C++ it is possible to add an option to always print primitive fields. Adding this option in the MessageToJson conversion makes protobuf add the parameters even though they are set to zero.

google::protobuf::util::JsonPrintOptions options;
options.always_print_primitive_fields = true;