Hi, I recently added unordered, stable collections to Boost.PolyCollection, internally based on boost::container::hub: * boost::base_unordered_collection * boost::function_unordered_collection * boost::any_unordered_collection * boost::variant_unordered_collection Tutorial: https://www.boost.org/doc/libs/develop/doc/html/poly_collection/tutorial.htm... Full reference: https://www.boost.org/doc/libs/develop/doc/html/poly_collection/reference.ht... If you're interested, please play with these new collections and provide your feedback before official shipping (Boost 1.93). Available at https://github.com/boostorg/poly_collection (both develop and master). Thank you! Also, I wrote an article describing the internal design of the library: https://bannalia.blogspot.com/2026/09/inside-boostpolycollection.html Best regards, Joaquín M López Muñoz