mongo::DBClientReplicaSet connection("replicaName", hosts, 0);
    const bool connected = connection.connect();    
 
    for( int i=0; i<1000; ++i)
    {
        mongo::BSONObj query = BSON( "fieldA" << i );
        mongo::Query mQuery = mongo::Query( query );
        mongo::BSONArray bsonTags;
        mQuery.readPref( mongo::ReadPreference_PrimaryPreferred, bsonTags );
        std::auto_ptr<mongo::DBClientCursor> cursor = connection.query( "database.collection",
                                                 mQuery,
                                                 0, 0, 0,
                                                 mongo::QueryOption_SlaveOk, 0 );
        // If there has been an election in the replica set, the cursor could be NULL. 
        if (cursor.get() != NULL){
            while(cursor->more()){
                mongo::BSONObj p = cursor->next();
                std::cout << p.getStringField("fieldB")<< std::endl;
            }
        }
        else {
            std::cout<< "cursor is NULL, handle error" <<std::endl;
        }
    }