Calculations

[[nodiscard]] double CalcTotalOrder(const std::vector<OrderRecord>& records, 
                                    const Date& startDate, const Date& endDate)
{
    return std::accumulate(std::begin(records), std::end(records), 0.0, 
        [&startDate, &endDate](double val, const OrderRecord& rec) {
            if (rec.CheckDate(startDate, endDate))
                return val + rec.CalcRecordPrice();
            else
                return val;
        }
    );
}