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

set — sorted. unordered_set — hash-based.

std::set:

• Sorted unique elements.
• O(log n).

1std::set<int> s = {3, 1, 2};
2// Sorted: 1, 2, 3

std::unordered_set:

• Hash-based.
• O(1) average.

1std::unordered_set<int> us = {3, 1, 2};
2// Unordered

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