std::set is an associative container that contains a sorted set of unique objects of type Key. Like std::map, it's usually implemented as a balanced binary search tree.

Use std::set when you need to store a collection of unique elements and keep them sorted at all times.

• Insertion: $O(\log N)$
• Search: $O(\log N)$
• Removal: $O(\log N)$

cpp Standard
#include <set>
#include <iostream>

std::set<int> st;
st.insert(10);
st.insert(20);
st.insert(10); // Not inserted (strictly unique)

std::cout << st.size(); // 2
for (int x : st) std::cout << x << " "; // Prints 10 20

• Trying to change an element value while it's in the set—elements are const. You must remove and re-insert.
• Thinking std::set uses a hash table (that's std::unordered_set).
• Overusing it when order doesn't matter (unordered versions are faster).