saved 2025-01-04-Joaquins2024Q4Update.md draft

Author: joaquintides

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+---
+layout: post
+nav-class: dark
+categories: joaquin
+title: Joaquín's Q4 2024 Update
+author-id: joaquin
+author-name: Joaquín M López Muñoz
+---
+
+During Q4 2024, I've been working in the following areas:
+
+### Boost.Mp11
+
+* Implemented [`mp_lambda`](https://www.boost.org/libs/mp11/doc/html/mp11.html#lambda)
+(released in Boost 1.87.0), a metaprogramming utility inspired by the venerable
+[_placeholder expressions_ from Boost.MPL](https://live.boost.org/libs/mpl/doc/refmanual/placeholder-expression.html).
+`mp_lambda` allows us to generate complex types specified with a rather natural syntax
+in terms of elementary input types indicated by
+[Boost.Mp11 _placeholder types_](https://www.boost.org/libs/mp11/doc/html/mp11.html#1_9).
+For instance, `std::pair<_1*, _2*>` can be regarded as a type template with
+two placeholder positions, and `mp_lambda<std::pair<_1*, _2*>>::fn<int, char>` is,
+unsurprisingly enough, the type `std::pair<int*, char*>`. My original motivation
+for writing this utility is to provide a Boost.Mp11 equivalent to Boost.MPL lambda
+expressions that can pave the way for an eventual modernization of
+Boost.Flyweight, which [relies heavily](https://www.boost.org/libs/flyweight/doc/tutorial/configuration.html#factory_types)
+on this functionality from Boost.MPL.
+* Rewritten the implementation of `mp_is_set`
+([PR#100](https://github.com/boostorg/mp11/pull/100), released in Boost 1.87.0) to
+achieve some rather noticeable compile-time improvements.
+
+### Boost.PolyCollection
+
+* Added the new [`boost::variant_collection`](https://www.boost.org/doc/libs/develop/doc/html/poly_collection/reference.html#poly_collection.reference.header_boost_poly_collection_va0)
+container (to be released in Boost 1.88.0).
+`boost::variant_collection_of<Ts...>` is similar to
+`std::vector<std::variant<Ts...>>`, with the crucial difference that elements
+storing the same alternative type are grouped together. For instance, the following:
+```cpp
+boost::variant_collection_of<int, double, std::string> c;
+// ...
+for(const auto& x:c) {
+  visit(
+    [](const auto& x) { std::cout << x << "\n"; },
+    x);
+}
+```
+will print first the `int`s in the collection, then the `double`s, and finally
+the `std::string`s. In exchange for this restriction, important
+[processing speedups](https://www.boost.org/doc/libs/develop/doc/html/poly_collection/performance.html#poly_collection.performance.processing_tests.results_for_boost_variant_collec)
+can be obtained.