// using C++11 range-based for loop
SubreadLengthQuery query(500, Compare::GREATER_THAN_EQUAL, dataset);
for (const BamRecord& r : query) {
    assert((r.QueryEnd() - r.QueryStart()) >= 500);  
}

// OR

// using iterators directly
SubreadLengthQuery query(500, Compare::GREATER_THAN_EQUAL, dataset);
auto iter = query.cbegin();
auto end  = query.cend();
for (; iter != end; ++iter) {
    assert((iter->QueryEnd() - iter->QueryStart()) >= 500);
}