struct    Event
{
    /*event id*/
    unsigned int m_nID;

    /*event priority*/
    EEventPriority    m_ePrior;

    /*type of events*/
    EEventTypes    m_evTypes;

    /*busy flag*/
    bool        m_bBusy;

    /*startup event time*/
    time_t        m_startup_time;

    /*expired event time*/
    time_t        m_expired_time;
};

//------------------------------------------------------------------------------
struct    nID{};
struct    ePrior{};

typedef    boost::multi_index::multi_index_container<
    Event,
    boost::multi_index::indexed_by
    <
        boost::multi_index::ordered_unique
        <
            boost::multi_index::tag<nID>, BOOST_MULTI_INDEX_MEMBER(Event, unsigned int, m_nID)
        >,
        boost::multi_index::ordered_non_unique
        <
            boost::multi_index::tag<ePrior>, BOOST_MULTI_INDEX_MEMBER(Event, EEventPriority, m_ePrior)
        >
    >
> eventsPool;

typedef eventsPool::index<nID>::type    nID_sub_arr;
typedef eventsPool::index<ePrior>::type ePrior_sub_arr;

ePrior_sub_arr::iterator itr = m_eventsPool.get<ePrior>().begin();
(*itr).m_bBusy = true; //error C3892: 'itr' : you cannot assign to a variable that is const