unordered set intersection in C++

Asymptotically, your algorithm is as good as it can get.

In practice, I'd add a check to loop over the smaller of the two sets and do lookups in the larger one. Assuming reasonably evenly distributed hashes, a lookup in a std::unoredered_set takes constant time. So this way, you'll be performing fewer such lookups.

You can do it with std::copy_if()

std::copy_if(a.begin(), a.end(), std::inserter(c, c.begin()), [b](const int element){return b.count(element) > 0;} );

Your algorithm is as good as it gets for a unordered set. however if you use a std::set (which uses a binary tree as storage) or even better a sorted std::vector, you can do better. The algorithm should be something like:

1. get iterators to a.begin() and b.begin()
2. if the iterators point to equal element add to intersection and increment both iterators.
3. Otherwise increment the iterator pointing to the smallest value
4. Go to 2.

Both should be O(n) time but using a normal set should save you from calculating hashes or any performance degradation that arises from hash collisions.