Boost.Unordered offers a catalog of hash containers with different standards compliance levels, performances and intented usage scenarios:

boost::unordered_set boost::unordered_map boost::unordered_multiset boost::unordered_multimap

Fully conformant implementations of std::unordered_[multi](set|map), but faster and up to the latest revisions of the standard even if you're working in an older version of C++ (heterogeneous lookup, try_emplace, contains, etc.)

boost::unordered_flat_set boost::unordered_flat_map

The fastest of the lot. Based on open addressing, these containers slightly deviate from the standard in exchange for top performance.

boost::unordered_node_set boost::unordered_node_map

Variations of boost::unordered_flat_(set|map) providing pointer stability.

boost::concurrent_flat_set boost::concurrent_flat_map

High performance for multithreaded scenarios. Introducing a new non-standard, iterator-free API.

• Online documentation
• Some benchmarks
• Technical articles on Boost.Unordered internal design:
  • Advancing the state of the art for std::unordered_map implementations
  • Inside boost::unordered_flat_map
  • Inside boost::concurrent_flat_map
  • Bulk visitation in boost::concurrent_flat_map

Boost.Unordered can be installed in a number of ways:

• Download Boost and you're ready to go (this is a header-only library requiring no building).
• Using Conan 2: In case you don't have it yet, add an entry for Boost in your conanfile.txt (the example requires at least Boost 1.83):

[requires]
boost/[>=1.83.0]

If you're not using any compiled Boost library, the following will skip building altogether:

[options]
boost:header_only=True

• Using vcpkg: Execute the command

vcpkg install boost-unordered

• Using CMake: Boost CMake support infrastructure allows you to use CMake directly to download, build and consume all of Boost or some specific libraries.

• Join the #boost-unordered discussion group at cpplang.slack.com (ask for an invite if you’re not a member of this workspace yet)
• Ask in the Boost Users mailing list (add the [unordered] tag at the beginning of the subject line)
• File an issue

• Pull requests against develop branch are most welcome. Note that by submitting patches you agree to license your modifications under the Boost Software License, Version 1.0.