The handler can be run in Log replaying mode, in that mode it will process packets that were recorded in a log file before.

class MyListener :
    public ErrorListener
    , public WarningListener
    , public ReplayListener
    , public StateChangesFeedListener
    , public TickerFeedListener
    , public MarketDataFeedListener
{
...
};

        HandlerSettings settings;
        settings.licenseDirectory = "../../license";
        settings.logDirectory     = "logs";
        settings.logLevel         = LogLevel::Debug;
        settings.templateFile = "Xetra Release 16.0 - Xetra market Data Interface - Final Version Fast Message Template.xml";

        ReplayOptions replayOptions;
        replayOptions.logs.push_back ("logToReplay/log.log");
        replayOptions.listener = &listener;

        Handler handler (settings);
        handler.registerErrorListener (&listener);
        handler.registerWarningListener (&listener);
        handler.registerStateChangesFeedListener (&listener);
        handler.registerTickerFeedListener (&listener);
        handler.registerMarketDataFeedListener (&listener);

        clog << "Will start the Handler in replay log mode ..." << endl;
        handler.start (replayOptions);

Note: More information can be found in Backtesting sample