flat_hash_map

If you don't need all the guarantees provided by std::unordered_map (pointer stability is usually the big one), you can go a /lot/ faster with a map that uses open addressing. Some of my favorite alternative hash map implementations are ska::flat_hash_map and ska::bytell_hash_map from https://github.com/skarupke/flat_hash_map. They're fast, and the single header implementation makes them easy to add to a project. For my use cases they generally offer similar performance to abseil and folly F14. Don't be fooled by the fact that they haven't been updated in ~5 years. I've been using them for nearly that long and have yet to find any bugs.

I've been using the author's flat_hash_map (https://github.com/skarupke/flat_hash_map) for several years now and have been really impressed. I've yet to find a single bug, it's nearly as fast as folly's hash maps (for my use case anyway) but far easier to integrate than folly.

You can look at the source in his github repository: https://github.com/skarupke/flat_hash_map