andrewg

  message Config {
    repeated Server server = 1;
  }

  message Server {
    string address = 1;
    int32 port = 2;
    bool standby = 3;
  }

  # main instance
  server { address: "127.0.0.1" port: 4567 }
  # backup instance
  server { address: "127.0.0.1" port: 9876 standby: true }

  Config config;
  google::protobuf::TextFormat::ParseFromString(input, &config);

  std::variant<int, bool, double> options;
  options = true;
  
  bool value = std::get<bool>(options);
  bool has_bool = std::holds_alternative<bool>(options);
  
  // or test which alternative is held
  if (auto i = std::get_if<int>(&options)) {
    // do something with int
  } else if (auto b = std::get_if<bool>(&options)) {
    // do something with bool
  } else {
    // do something with double
  }