You haven’t provided an output iterator for set_intersection
template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator set_intersection ( InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result );
Fix this by doing something like
...;
set<int> intersect;
set_intersection(s1.begin(), s1.end(), s2.begin(), s2.end(),
std::inserter(intersect, intersect.begin()));
You need a std::insert
iterator since the set is as of now empty. We cannot use std::back_inserter
or std::front_inserter
since set doesn’t support those operations.