C++: What is the difference between std::set and std::unordered_set?

set — sorted unique elements. unordered_set — hash set.

std::set:

• Sorted (red-black tree).
• O(log n) operations.
• Ordered iteration.

1std::set<int> s = {3, 1, 4, 1, 5};
2// Iterates: 1, 3, 4, 5 (sorted, unique)

std::unordered_set:

• Unsorted (hash table).
• O(1) average.
• Unordered iteration.

1std::unordered_set<int> us = {3, 1, 4, 1, 5};
2// Iterates: 3, 1, 4, 5 (any order, unique)

When to use:

• set — need sorted order.
• unordered_set — fast lookup.