Variant based state machine

variant-fsm.cpp

// state machine that handes processing of data
struct Fetch {};
struct Stop {};
struct StartProcessing {
    SamplePtr sample;
};
struct WaitProcessed {
   SamplePtr sample;
   std::string json_path;
};
struct FinishProcessing {
    SamplePtr sample;
    std::string json_path;
    bool ok;
};
struct Reconstruct {
    SamplePtr sample;
};
struct Send {
    SamplePtr sample;
};

using State = std::variant<Fetch, StartProcessing, WaitProcessed, FinishProcessing, Reconstruct, Send, Stop>;

State handle(Fetch &s);
State handle(StartProcessing &s);
State handle(WaitProcessed &s);
State handle(FinishProcessing &s);

State handle(Reconstruct &s) {
    reconstruct(s.sample);
    return Send{s.sample};
}

State handle(Send &s) {
    send(std::move(s));
    return Fetch{};
}

State handle(Stop &s) {
    return s;
}

void run() {
    State state = Fetch{};
    while (!std::holds_alternative<Stop>(state)) {
        state = std::visit([=](auto &a) { return handle(a); }, state);
    }
}