C++: What is std::flat_set and its performance characteristics?

1#include <flat_set>
2
3std::flat_set<int> fs;
4
5// Insert
6fs.insert(3);
7fs.insert(1);
8fs.insert(4);
9fs.insert(1);  // Ignored (duplicate)
10// fs = {1, 3, 4}
11
12// Search: O(log n)
13if (fs.contains(3)) { /* found */ }
14auto it = fs.find(3);
15
16// Lower/upper bound
17auto lb = fs.lower_bound(2);  // Points to 3
18auto ub = fs.upper_bound(2);  // Points to 3
19
20// Erase
21fs.erase(3);
22
23// Iteration (sorted)
24for (int x : fs) {
25    std::cout << x << " ";
26}
27
28// Capacity
29std::cout << fs.size();       // Number of elements
30std::cout << fs.capacity();   // Allocated space
31std::cout << fs.empty();      // Is empty
32
33// Underlying vector
34const auto& v = fs.containers();