What is the difference between set and hashset in C++ STL?

Question

When should I choose one over the other? Are there any pointers that you would recommend for using the right STL containers?

Answer 1

I don't think anyone has answered the other part of the question yet.

The reason to use hash_set or unordered_set is the usually O(1) lookup time. I say usually because every so often, depending on implementation, a hash may have to be copied to a larger hash array, or a hash bucket may end up containing thousands of entries.

The reason to use a set is if you often need the largest or smallest member of a set. A hash has no order so there is no quick way to find the smallest item. A tree has order, so largest or smallest is very quick. O(log n) for a simple tree, O(1) if it holds pointers to the ends.