Best way to encapsulate "optional" fields within a struct generically in C++?

Question

I have many concrete-structs and I want to designate fields as optional (present or not-present). Just wondering what ideas people have for achieving this. Here is an example struct (fields can be other structs as well, even vectors of structs):

 struct LogonMessage_t
       {
          Header_t header; // this points to another struct containing all primitives
          std::string username;
          std::string password;
          std::vector<LogonOption_t> LogonOptions;
          int subaccountid;
          std::string Text;
       }

I'd like to default all fields to not-present and enable them one by one, perhaps in their setters. Since these structs are generated, a generic method would be preferable.

My ideas so far are:

1. Bitmap of fields indicating if a field is set or not
2. Use sentinel values ("\0" for std::string, -1 for int, -1.0f for float
3. Some kind of template container/proxy encapsulating each field to indicate if it's set or not, any ideas? I think this might be the best approach.

Btw, using maps or other STL containers to encapsulate the fields won't work here, they need to be concrete structs.

Answer 1

Sounds like you want boost.optional.

Answer 2

Keep it simple. Use a flag member variable that you can set by or-ing constants together and inspect by and-ing them.

Problem with sentinel values is choosing ones that are not also legal field values (now and in the future).