Key-value stores are one of the simplest forms of database. - Implementing a Key-Value Store

Benchmark

• Comprehensive C++ Hashmap Benchmarks 2022
  • 3 years ago absl::flat_hash_map was one of the fastest maps. It still is quite fast, and seems to perform especially well for large maps. - Abseil Containers
• Finding the Fastest, Memory Efficient Hashmap - 2020 - results
  • hashmap benchmarks / r/cpp - benchmark code
• Benchmark of major hash maps implementation - 2017

Conclusion

Hashes

This summary is simple: in all benchmarks where hashing was involved, robin_hood::hash is overall the fastest. Only for long strings absl::Hash is faster. Having said that, absl::Hash has a lot more features and is backed by the power of Google, so it might be worth considering anyways.

I can only warn from the libstdc++-v3 default implementation for integral types. Only use it if you are absolutely sure about what data you are about to insert.

Hashmaps

If you are dealing with small maps and integral data types as keys, emilib1::HashMap might be fastest. But be aware that I cannot vouch for it’s stability. It is very new, and while integrating it the author still had to fix a few kinks to get it working in my benchmark. If that does not bother you, it’s search performance is very fast.

If you want a more stable map with very good performance, I’d go for absl::flat_hash_map as the default map. It is very stable, has lots of features, and definitely well tested.

If your map is modified with inserts and removals a lot, tsl::robin_map is fastest. It has high memory usage though. robin_hood::unordered_flat_map is only slightly slower but has much less memory requirements.

If your key is relatively slow to hash and to compare like a std::string, robin_hood::unordered_node_map is the way to go. It’s memory usage is quite low and it is fastest. Integration is very easy, since it is a relatively small single header file.

Sometimes it is very important to iterate and process all entries in a map very fast, e.g. in games. In that case, tsl::sparse_map is the clear winner. Iteration is exceptionally fast, it uses little memory, and find & insert & erase performance is ok as well.

see also

• Super high performance C/C++ hash map (table, dictionary)
  • sparsehash - several hash-map implementations, similar in API to SGI’s hash_map class, but with different performance characteristics.