iterator begin(ReadModifyWriteOption rmw=
    ReadModifyWriteOption::no_read_modify_write(), bool readonly=false,
    BulkRetrievalOption bulkretrieval=
        BulkRetrievalOption::no_bulk_retrieval(),
    bool directdb_get=true)
 
        Begin a read-write or readonly iterator which sits on the first key/data pair of the database.
const_iterator begin(BulkRetrievalOption bulkretrieval=
    BulkRetrievalOption::no_bulk_retrieval(),
    bool directdb_get=true) const
 
        Begin a read-only iterator.
The parameters in begin functions of this group have identical meaning to thoes in db_vector::begin , refer to those functions for details.