We've been doing this for 50 years, you don't need a class hierarchy for your event struct.

  enum EventType {
      EVENT_NONE,
      EVENT_TEMPERATURE_READING,
      EVENT_SMOKE_DETECTED,
      EVENT_CARBON_MONOXIDE_DETECTED,
  };
  struct Event {
      EventType type;
      union {
          TemperatureDeadingData temperature_reading;
          SmokeDetectedData smoke_detected;
          CarbonMonoxideDetected carbon_monoxide_detected
      } data;
  };

  std::vector<Event> events;