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

问题

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.

回答1:

Sounds like you want boost.optional.

回答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).

回答3:

Both string and vector has an empty default state, which you can test for with if (username.empty()) etc.

For a subaccountid I would guess that 0 would be a similar empty value. Otherwise -1 could be ok.

回答4:

I'd use one a flag. I can propose you two method, one keeping the value on the heap and one on the stack.

In the first you use a std::pair and the first field is the existence-flag. The second approach is via a boost::shard_ptr, if pointer points to 0 the field is not existing.

In both the cases my advice is not to access directly to the element in Value but using instead a couple of functions. const Value& value() const { return } Value& value() { return }

francesco

来源：https://stackoverflow.com/questions/5379102/best-way-to-encapsulate-optional-fields-within-a-struct-generically-in-c