Question 1

How much faster is robin_hood compared to std::unordered_map?

Accepted Answer

Benchmarks indicate robin_hood can be 2x or more faster with lower memory overhead, but gains vary based on workload and hash quality. Real-world tests show significant improvements in lookup and insertion times.

Question 2

Is robin_hood hash map thread-safe for concurrent access?

Accepted Answer

No, it lacks built-in thread safety; concurrent modifications require external synchronization like mutexes, similar to std::unordered_map. This is a trade-off for its performance-focused design.

Question 3

Should I use robin_hood or ankerl::unordered_dense for a new C++ project?

Accepted Answer

The author recommends ankerl::unordered_dense as the successor, which likely offers improvements and active maintenance. For new projects, unordered_dense is generally better unless you have legacy dependencies on robin_hood.

Question 4

How to choose between unordered_flat_map and unordered_node_map?

Accepted Answer

Use unordered_flat_map for maximum speed when element references don't need to stay stable after modifications. Choose unordered_node_map if you require pointers or references to remain valid, accepting slower access due to indirection.

Question 5

Can I use custom hash functions with robin_hood?

Accepted Answer

Yes, but ensure they are high-quality to avoid performance issues or overflow errors. The library defaults to robin_hood::hash for optimized hashing, falling back to std::hash for other types.

Question 6

How to integrate robin_hood into a CMake project using Conan?

Accepted Answer

Add robin-hood-hashing to your conanfile.txt, use the CMake generator, and link the CONAN_PKG target in your CMakeLists.txt. The README provides step-by-step instructions for seamless setup.

Question 7

What are the memory savings of robin_hood over std::unordered_map?

Accepted Answer

robin_hood often reduces memory usage by 50% or more due to compact storage and efficient allocation, especially for small key-value pairs. Benchmarks highlight this in scenarios with high element counts.

robin-hood-hashing

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