Quadratic Copying Performance Degradation

[cmazakas]

Inserting in "hash order" can dramatically degrade the performance of insertion. This can be easily triggered by:

unordered_flat_map<int, int> map1(...);
unordered_flat_map<int, int> map2;
for (auto const& kv: map1) { map2.insert(kv); }

For users, this can be circumvented easily by either using the copy constructor or by calling reserve() prior to insertion.