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

A hash_set would be implemented by a hash table, which has mostly O(1) operations, whereas a set is implemented by a tree of some sort (AVL, red black, etc.) which have O(log n) operations, but are in sorted order.

Edit: I had written that trees are O(n). That's completely wrong.

Answer 2

hash_set is an extension that is not part of the C++ standard. Lookups should be O(1) rather than O(log n) for set, so it will be faster in most circumstances.

Another difference will be seen when you iterate through the containers. set will deliver the contents in sorted order, while hash_set will be essentially random (Thanks Lou Franco).

Edit: The C++11 update to the C++ standard introduced unordered_set which should be preferred instead of hash_set. The performance will be similar and is guaranteed by the standard. The "unordered" in the name stresses that iterating it will produce results in no particular order.

Answer 3

Another thing to keep in mind is that with hash_set you have to provide the hash function, whereas a set only requires a comparison function ('<') which is easier to define (and predefined for native types).

Answer 4

stl::set is implemented as a binary search tree. hashset is implemented as a hash table.

The main issue here is that many people use stl::set thinking it is a hash table with look-up of O(1), which it isn't, and doesn't have. It really has O(log(n)) for look-ups. Other than that, read about binary trees vs hash tables to get a better idea of the data structures.

Answer 5

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.