返回任何/未知类型 c++

Question

I am attempting to implement msgpack (a json alternative) in c++, and am creating an Iterator class to loop over my byte array, which is separated based on certain headers. So how would the iterator construct and return any type, Eg. vector<int> , map<int, string> etc. Is std::any or std::variant the only way to do this? (I don't want to use boost).

My code so far https://github.com/t348575/msgpack

Answer 1

There are different approaches for what you want to do and each one has its own requirements and drawbacks.

Solution 1: The user knows at compiletime what the serialized data looks like Just write a template function that takes a type and try do deserialize into it. Check the runtime type info of the binary blob and throw a exception if the type does not match.

Solution 2: User knows the set of types that are possible, std::variant approach: Make use of the visitor pattern. You can look at the interface of variant for details.

Solution 3: The user has no clue of what types are to come: You are pretty much out of luck here. You need to make something like std::any which is in its essence a glorified void*.

Answer 2

C++ is designed to be a statically typed language. Hence what you're trying to accomplish (getting a function to return a type determined run-time) is expectedly a struggle:). I'd personally avoid this approach.

std::variant is one solution but it is only going to result in the client probing the content of each return value in a big case statement; or subsequently getting a callback from the visitor pattern. Neither is really the best way to get output from a parser.

I'd have a look at good C++ JSON parsers, see how they solve serialization/deserialization, eg https://github.com/nlohmann/json