message OneofMessage
{
    int32 testFieldInt = 1;
    oneof testOneof
    {
        int32 testOneofFieldInt = 42;
        ComplexMessage testOneofComplexField = 3;
        ComplexMessage testOneofSecondComplexField = 4;
    }
    oneof secondOneof
    {
        int32 secondFieldInt = 43;
        ComplexMessage secondComplexField = 5;
        ComplexMessage secondSecondComplexField = 6;
    }
}

// Existing protobuf enum
enum QtProtobufFieldEnum {
TestFieldIntProtoFieldNumber = 1,
TestOneofFieldIntProtoFieldNumber = 42,
TestOneofComplexFieldProtoFieldNumber = 3,
TestOneofSecondComplexFieldProtoFieldNumber = 4,
SecondFieldIntProtoFieldNumber = 43,
SecondComplexFieldProtoFieldNumber = 5,
SecondSecondComplexFieldProtoFieldNumber = 6,
};
Q_ENUM(QtProtobufFieldEnum)
  
// testOneof enum
enum testOneofField {
InvalidFieldNumber = QtProtobuf::InvalidFieldNumber,
testOneofFieldInt = 42,
testOneofComplexField = 3,
testOneofSecondComplexField = 4,
};
Q_ENUM(testOneofField)
// secondOneof enum
enum secondOneofField {
InvalidFieldNumber = QtProtobuf::InvalidFieldNumber,
secondFieldInt = 43,
secondComplexField = 5,
secondSecondComplexField = 6,
};
Q_ENUM(secondOneofField)

testOneofField testOneofField() const;
secondOneofField secondOneofField() const;

OneofMessage test = ...;
 switch (test.testOneofCase() /* getter for a oneof field named 'testOneof' */) {
   case OneofMessage::testOneofCase::None:
     // no value
     break;
   case OneofMessage::testOneofCase::testOneofComplexField:
     // the contained 'ComplexField' field is initialized
     ...
   // etc...
 }