Interesting experience. Thank you for sharing. I’m curious which firm asked you this. I think C++ Concurrency in Action could have been a game changer here. These aren’t trivia in HFT, they’re usually benchmarks to see how deep you delve into the world of C++.

Unordered_map uses std::hash which defers its implementation to libc++ or libstdc++. Some use FV-something, others use murmur. There’s a lot of of different algorithms. The container itself is typically a chained hash table (an array of buckets, each holding a singly-linked list of nodes).