Unordered polymorphic collections

Portrait of Joaquín M López Muñoz Joaquín M López Muñoz · Oct 2, 2026

During Q3 2026, I’ve been working in the following areas:

Unordered polymorphic collections

The preexisting containers offered by Boost.PolyCollection (boost::base_collection, boost::function_collection, boost::any_collection, boost::variant_collection) do not provide full control over the global positioning of an element, for the simple reason that each element goes to its dedicated, type-specific segment. At the segment level, though, users can decide where an element is inserted much as they would with a std::vector, which is, ultimately, the data structure these segments are based on.

If the user can dispense with intra-segment positioning, segments can be implemented with a different internal data structure. Unordered polymorphic collections (boost::base_unordered_collection, boost::function_unordered_collection, boost::any_unordered_collection, boost::variant_unordered_collection) internally use boost::container::hub-based segments, giving these collections iterator and reference stability. Their performance profile relative to the existing ordered collections is different: insertion is on par or even faster, whereas iteration is definitely slower (nothing beats iterating over a vector). My hypothesis is that iterator stability is a very desirable property in the kind of scenarios targeted by Boost.PolyCollection — time will tell. Unordered polymorphic collections will ship with Boost 1.93.

The internal design of Boost.PolyCollection is interesting: all eight collections are instantiations of the same internal poly_collection class template, parameterized by a so-called model that encodes:

  • The type of runtime polymorphism used (OOP, function wrapping, duck typing, std::variant-like).
  • Whether the collection is ordered or unordered.
  • Whether the collection is closed (boost::variant_[unordered]_collection) or open (the rest).

The result is a rich internal structure in which interoperable parts are combined across three independent dimensions. As a reference for interested readers (and for my future self) I’ve written the article “Inside Boost.PolyCollection”, which explains the design in some detail.

Boost.PolyCollection maintenance

boost::container::hub

  • Optimized range insertion (PR#339).
  • Written maintenance fix PR#340.

Boost.MultiIndex

Papergate

(Not related to Boost.) Papergate is an AI tool that takes any WG21 proposal and determines whether the paper itself answers a simple question: why is this worth standardizing? Papergate looks at things such as the presentation of alternatives, cost/benefit analysis, availability of a reference implementation, etc. The goal is not to assess the merits of a proposal, but merely to check whether the paper addresses the basic questions a human reviewer will ask before digging into the details. I’ve been working on the md prompt powering Papergate. Papergate is integrated into wg21.org.

Support to the community

  • I’ve been helping a bit with Mark Cooper’s very successful Boost Blueprint series on X.
  • Working on a classification of Boost libraries according to their maintenance status (work in progress). This will help us point new volunteers toward those libraries most in need of attention.
  • Supporting the community as a member of the Fiscal Sponsorship Committee (FSC).

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